Photocrosslinked biodegradable hydrogel

ABSTRACT

A photocrosslinked biodegradable hydrogel includes a plurality of natural polymer macromers cross-linked with a plurality of hydrolyzable acrylate cross-links. The hydrogel is cytocompatible and produces substantially non-toxic products upon degradation.

RELATED APPLICATION

This application claims priority from U.S. Provisional PatentApplication No. 61/141,266, filed Dec. 30, 2008, the entirety of whichis hereby incorporated by reference.

TECHNICAL FIELD

The present invention generally relates to tissue engineering, bioactivefactor delivery, and disease therapeutics, and more particularly to aphotocrosslinked biodegradable hydrogel.

BACKGROUND OF THE INVENTION

Hydrogels are used in biomedical applications, such as drug deliveryvehicles, cell encapsulation matrices, and tissue engineering scaffoldsbecause many of their physical properties are similar to natural tissue.Hydrogels are insoluble 3-D networks of physically or chemicallycross-linked hydrophilic polymers, which exhibit a high degree ofswelling in aqueous environments.

Alginate is a linear unbranched polysaccharide derived from seaweed thatcontains the repeating units of 1,4-linked β-D-mannuronic acid andα-L-guluronic acid. Alginates have reversible gelling properties inaqueous solutions related to the ionic interactions between divalentcations. Alginate can be formed into a hydrogel and used as a drugdelivery vehicle.

There is limited control over the mechanical properties, swellingratios, and degradation profiles of ionically cross-linked alginatehydrogels, which is likely due to the uncontrollable loss of divalentcations into the surrounding environment. Approaches to chemicallycrosslinking alginate microcapsules or macroscopic hydrogels may utilizeintermolecular covalent cross-linking rather than ionic cross-linking inorder to synthesize alginate hydrogels with a wide range of mechanicalproperties. However, the reagents and reaction conditions used with suchapproaches can be toxic to encapsulated cells or growth factors, andhydrogels covalently cross-linked are not biodegradable.

SUMMARY OF THE INVENTION

The present invention generally relates to tissue engineering, and moreparticularly to a photocrosslinked biodegradable hydrogel, a method forforming the hydrogel, and related methods for using the hydrogel indifferent tissue engineering application. The photocrosslinkedbiodegradable hydrogel can include a plurality of natural polymermacromers cross-linked with a plurality of degradable cross-links. Thehydrogel can be cytocompatible and produce substantially non-toxicproducts upon degradation.

Another aspect of the present invention relates to a method for forminga photocrosslinked biodegradable hydrogel. The method can includereacting at least one acryl group with at least one natural polymermacromer to form a mixture comprising a plurality of acrylatedmacromers. The mixture can then be contacted with a photoinitiator thatcan initiate cross-linking of the macromers. The mixture can be exposedto a light source to initiate cross-linking of the macromers and form aplurality of natural polymer macromers cross-linked with a plurality ofhydrolyzable acrylate cross-links.

A further aspect of the invention relates to a method for promotingtissue growth in a subject. The method can include administering aphotocrosslinked biodegradable hydrogel to a target site in the subject.The hydrogel can include a plurality of natural polymer macromerscross-linked with a plurality of hydrolyzable acrylate cross-links andat least one cell dispersed on or within the hydrogel. The hydrogel maybe cytocompatible and produce substantially non-toxic products upondegradation.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features of the present invention will becomeapparent to those skilled in the art to which the present inventionrelates upon reading the following description with reference to theaccompanying drawings, in which:

FIG. 1 is a schematic illustration showing a process for preparing aphotocrosslinked degradable hydrogel in accordance with one aspect ofthe present invention;

FIG. 2 is a series of fluorescent photographs of human MSCsphotoencapsulated with BMP-2 using ALG, RGD-ALG, HP-ALG, and RGD-HP-ALGhydrogels after 2-weeks of culture;

FIG. 3 is a schematic illustration showing a process for preparing anRGD-modified methacrylated alginate in accordance with an aspect of theinvention;

FIG. 4 is a schematic illustration showing the synthesis of an acrylatedRGD-modified alginate hydrogel in accordance with another aspect of theinvention;

FIGS. 5A-D are a series of plots showing: elastic modulus in compressionof photocrosslinked acrylated RGD+methacrylated alginate (ACR-RGD-ALG)or acrylated RGD+MA-Heparin+methacrylated alginate (ACR-RGD-HP-ALG)hydrogel after 24 hrs equilibration in deionized water or DMEM (10 mgacrylated RGD/g alginate) (FIG. 5A); equilibrium swelling ratios ofphotocrosslinked ALG and HP-ALG with various amount of acrylated RGDafter 24 hours equilibration in DMEM (FIG. 5B); swelling ratio change ofphotocrosslinked ACR-RGD-ALG or acrylated ACR-RGD-HP-ALG hydrogel inDMEM over time (FIG. 5C); and degradation of photocrosslinkedACR-RGD-ALG or acrylated ACR-RGD-HP-ALG hydrogel in DMEM over time (FIG.5D);

FIGS. 6A-B are a series of plots showing quantification of microbeadgradient along 10 cm (FIG. 8A) and 5 cm (FIG. 8B) hydrogel (5 mmsections);

FIG. 7 is a plot showing quantification of BMP-2 and TGF-β₁ gradients inalginate hydrogel;

FIG. 8 a schematic illustration showing a process for preparing amethacrylated heparin macromer;

FIG. 9 is a series of scanning electron micrographs of polyethyleneoxide (PEO) extracted electrospun pure-, RGD-, and heparin-RGD-modifiedmethacrylated alginate nanofiber in dH₂O at 37° C. for 3 weeks (imagesrepresent (a) 8PMAIg/4PEO, (b) 8RMAIg/4PEO, and (c) 8HRMAIg/4PEO);

FIG. 10 illustrates the swelling ratios of photocrosslinked alginatehydrogels over time. Values represent mean and standard deviation (n=3);

FIG. 11 is a plot comparing in vitro degradation of photocrosslinkedalginate hydrogels over time. Mass loss (%)=(W_(i)−W_(d))/W_(i)×100,where W_(i)=initial weight and W_(d)=weight after degradation. Valuesrepresent mean and standard deviation (n=3) (*p<0.05 with MAALG-14);

FIGS. 12A-B are plots comparing stress vs. strain (FIG. 12A) and elasticmoduli vs. time in compression of photocrosslinked MAALG-25 hydrogelsduring degradation (FIG. 12B) (*p<0.05);

FIG. 13 is a graph showing the quantified cell density of live and deadcells for a cell culture insert, methacrylated alginate, or aphotocrosslinked alginate hydrogel (cells were stained by FDA/EB. Greenand orange-red colors indicate viable and dead cells, respectively.Values represent mean and standard deviation (N=3). The scale barindicates 200 μm. *p<0.05, **p>0.05);

FIG. 14 is a series of fluorescence micrographs showing live (FDA) anddead (EB) encapsulated bovine chondrocytes in various photocrosslinkedalginate hydrogels cultured in vitro for 7 days (scale bar indicates 200μm);

FIGS. 15A-E are a series of plots showing elastic moduli in compression(FIG. 15A), swelling ratio changes in DMEM (FIG. 15B) and diH₂O (FIG.15C), and in vitro degradation of photocrosslinked HP-ALG hydrogels andalginate hydrogels in DMEM (FIG. 15D) and diH₂O (FIG. 15E). Mass loss(%)=(W_(i)−W_(d))/W_(i)×100, where W_(i)=initial weight and W_(d)=weightafter degradation (values represent mean and standard deviation; *p<0.05compared to HP-ALG);

FIGS. 16A-D are a series of plots showing TGF-β₁ (FIG. 16A), FGF-2 (FIG.16B), VEGF (FIG. 16C), and BMP-2 (FIG. 16D) released fromphotocrosslinked HP-ALG hydrogels and alginate hydrogels (valuesrepresent mean±standard deviation; *p<0.05);

FIGS. 17A-B are plots showing the bioactivities of VEGF (FIG. 17A) andBMP-2 (FIG. 17B) released from the delivery systems, as assessed bymeasuring HUVEC proliferation and ALP activity of MC3T3 preosteoblastscultured with the delivery systems, respectively (p<0.05 compared toEGM-2 or VEGF in alginate hydrogel group; *p<0.05 compared to onlyHP-ALG hydrogel or DMEM group);

FIGS. 18A-B are a series of microscopic photographs showing H&E andGoldner's trichrome-stained histological sections of BMP-2 deliverysystem implants at 8 weeks (FIG. 18A) and H&E staining at highmagnification of the BMP-2-loaded HP-ALG implant at 8 weeks (FIG. 18B).The mature bone in FIG. 23B contained viable osteocytes (closedtriangle) (photographs in FIG. 18A were taken at the samemagnification);

FIGS. 19A-B are a series of graphs showing bone formation vs. implanttype (FIG. 19A) and calcium concentration vs. implant type (FIG. 19B) at8 weeks after implantation (values represent mean±standard deviation;*p<0.05);

FIGS. 20A-D are a series of plots showing stress vs. strain (FIG. 20A),elastic moduli (FIG. 20B) in compression of RGD-modified and unmodifiedphotocrosslinked alginate hydrogels after 24 hours equilibrium in DMEM,swelling ratios (FIG. 20C), and in vitro degradation (FIG. 20D) ofRGD-modified and unmodified photocrosslinked alginate hydrogels in DMEM(*p<0.05);

FIGS. 21A-B are a series of plots showing swelling ratios (FIG. 22A) andin vitro degradation (FIG. 22B) vs. time of RGD-modified and unmodifiedphotocrosslinked alginate hydrogels in diH₂O (*p<0.05);

FIG. 22 is a plot showing cumulative release (%) profiles vs. time ofTGF-β₁ from RGD-modified and unmodified photocrosslinked alginatehydrogels (values represent mean±standard deviation);

FIG. 23 is a plot showing GAG/DNA contents vs. time for bovinechondrocytes encapsulated with TGF-β₁ in RGD-modified and unmodifiedphotocrosslinked alginate hydrogels cultured in DMEM and encapsulatedbovine chondrocytes without TGF-β₁ in RGD-modified and unmodifiedphotocrosslinked alginate hydrogels cultured in DMEM containing TGF-β₁for 2, 4, and 6 weeks (scale bar indicates 200 μm; *p<0.05).

DETAILED DESCRIPTION

Methods involving conventional molecular biology techniques aredescribed herein. Such techniques are generally known in the art and aredescribed in detail in methodology treatises, such as Current Protocolsin Molecular Biology, ed. Ausubel et al., Greene Publishing andWiley-Interscience, New York, 1992 (with periodic updates). Unlessotherwise defined, all technical terms used herein have the same meaningas commonly understood by one of ordinary skill in the art to which thepresent invention pertains. Commonly understood definitions of molecularbiology terms can be found in, for example, Rieger et al., Glossary ofGenetics: Classical and Molecular, 5th Ed., Springer-Verlag: New York,1991, and Lewin, Genes V, Oxford University Press: New York, 1994. Thedefinitions provided herein are to facilitate understanding of certainterms used frequently herein and are not meant to limit the scope of thepresent invention.

In the context of the present invention, the term “bioactive agent” canrefer to any agent capable of promoting tissue formation, destruction,and/or targeting a specific disease state. Examples of bioactive agentscan include, but are not limited to, chemotactic agents, variousproteins (e.g., short term peptides, bone morphogenic proteins,collagen, glycoproteins, and lipoprotein), cell attachment mediators,biologically active ligands, integrin binding sequence, various growthand/or differentiation agents and fragments thereof (e.g., epidermalgrowth factor (EGF), hepatocyte growth factor (HGF), vascularendothelial growth factors (VEGF), fibroblast growth factors (e.g.,bFGF), platelet derived growth factors (PDGF), insulin-like growthfactor (e.g., IGF-I, IGF-II) and transforming growth factors (e.g.,TGF-β I-III), parathyroid hormone, parathyroid hormone related peptide,bone morphogenic proteins (e.g., BMP-2, BMP-4, BMP-6, BMP-7, BMP-12,BMP-13, BMP-14), transcription factors, such as sonic hedgehog, growthdifferentiation factors (e.g., GDF5, GDF6, GDF8), recombinant humangrowth factors (e.g., MP52 and the MP-52 variant rhGDF-5),cartilage-derived morphogenic proteins (CDMP-1, CDMP-2, CDMP-3), smallmolecules that affect the upregulation of specific growth factors,tenascin-C, hyaluronic acid, chondroitin sulfate, fibronectin, decorin,thromboelastin, thrombin-derived peptides, heparin-binding domains,heparin, heparan sulfate, polynucleotides, DNA fragments, DNA plasmids,MMPs, TIMPs, interfering RNA molecules, such as siRNAs,oligonucleotides, proteoglycans, glycoproteins, glycosaminoglycans, andDNA encoding for shRNA.

As used herein, the terms “biodegradable” and “bioresorbable” may beused interchangeably and refer to the ability of a material (e.g., anatural polymer or macromer) to be fully resorbed in vivo. “Full” canmean that no significant extracellular fragments remain. The resorptionprocess can involve elimination of the original implant material(s)through the action of body fluids, enzymes, cells, and the like.

As used herein, the term “carrier material” can refer to a materialcapable of transporting, releasing, and/or complexing at least onebioactive agent.

As used herein, the term “function and/or characteristic of a cell” canrefer to the modulation, growth, and/or proliferation of at least onecell, such as a progenitor cell and/or differentiated cell, themodulation of the state of differentiation of at least one cell, and/orthe induction of a pathway in at least one cell, which directs the cellto grow, proliferate, and/or differentiate along a desired pathway,e.g., leading to a desired cell phenotype, cell migration, angiogenesis,apoptosis, etc.

As used herein, the term “macromer” can refer to any natural polymer oroligomer.

As used herein, the term “polynucleotide” can refer to oligonucleotides,nucleotides, or to a fragment of any of these, to DNA or RNA (e.g.,mRNA, rRNA, siRNA, tRNA) of genomic or synthetic origin which may besingle-stranded or double-stranded and may represent a sense orantisense strand, to peptide nucleic acids, or to any DNA-like orRNA-like material, natural or synthetic in origin, including, e.g.,iRNA, ribonucleoproteins (e.g., iRNPs). The term can also encompassnucleic acids (i.e., oligonucleotides) containing known analogues ofnatural nucleotides, as well as nucleic acid-like structures withsynthetic backbones.

As used herein, the term “polypeptide” can refer to an oligopeptide,peptide, polypeptide, or protein sequence, or to a fragment, portion, orsubunit of any of these, and to naturally occurring or syntheticmolecules. The term “polypeptide” can also include amino acids joined toeach other by peptide bonds or modified peptide bonds, i.e., peptideisosteres, and may contain any type of modified amino acids. The term“polypeptide” can also include peptides and polypeptide fragments,motifs and the like, glycosylated polypeptides, and all “mimetic” and“peptidomimetic” polypeptide forms.

As used herein, the term “cell” can refer to any progenitor cell, suchas totipotent stem cells, pluripotent stem cells, and multipotent stemcells, as well as any of their lineage descendant cells, including moredifferentiated cells. The terms “stem cell” and “progenitor cell” areused interchangeably herein. The cells can derive from embryonic, fetal,or adult tissues. Examples of progenitor cells can include totipotentstem cells, multipotent stem cells, mesenchymal stem cells (MSCs),hematopoietic stem cells, neuronal stem cells, hematopoietic stem cells,pancreatic stem cells, cardiac stem cells, embryonic stem cells,embryonic germ cells, neural crest stem cells, kidney stem cells,hepatic stem cells, lung stem cells, hemangioblast cells, andendothelial progenitor cells. Additional exemplary progenitor cells caninclude de-differentiated chondrogenic cells, chondrogenic cells, cordblood stem cells, multi-potent adult progenitor cells, myogenic cells,osteogenic cells, tendogenic cells, ligamentogenic cells, adipogeniccells, and dermatogenic cells.

As used herein, the term “subject” can refer to any animal, including,but not limited to, humans and non-human animals (e.g., rodents,arthropods, insects, fish (e.g., zebrafish)), non-human primates,ovines, bovines, ruminants, lagomorphs, porcines, caprines, equines,canines, felines, ayes, etc.), which is to be the recipient of aparticular treatment. Typically, the terms “patient” and “subject” areused interchangeably herein in reference to a human subject.

As used herein, the term “tissue” can refer to an aggregate of cellshaving substantially the same function and/or form in a multicellularorganism. “Tissue” is typically an aggregate of cells of the sameorigin, but may be an aggregate of cells of different origins. The cellscan have the substantially same or substantially different function, andmay be of the same or different type. “Tissue” can include, but is notlimited to, an organ, a part of an organ, bone, cartilage, skin, neuron,axon, blood vessel, cornea, muscle, fascia, brain, prostate, breast,endometrium, lung, pancreas, small intestine, blood, liver, testes,ovaries, cervix, colon, stomach, esophagus, spleen, lymph node, bonemarrow, kidney, peripheral blood, embryonic, or ascite tissue.

As used herein, the terms “inhibit,” “silencing,” and “attenuating” canrefer to a measurable reduction in expression of a target mRNA (or thecorresponding polypeptide or protein) as compared with the expression ofthe target mRNA (or the corresponding polypeptide or protein) in theabsence of an interfering RNA molecule of the present invention. Thereduction in expression of the target mRNA (or the correspondingpolypeptide or protein) is commonly referred to as “knock-down” and isreported relative to levels present following administration orexpression of a non-targeting control RNA.

As used herein, the term “aggregate” can refer to a group or clustercomprising at least two or more cells (e.g., progenitor and/ordifferentiated cells).

As used herein, the term “population” can refer to a collection ofcells, such as a collection of progenitor and/or differentiated cells.

As used herein, the term “differentiated” as it relates to the cells ofthe present invention can refer to cells that have developed to a pointwhere they are programmed to develop into a specific type of cell and/orlineage of cells. Similarly, “non-differentiated” or “undifferentiated”as it relates to the cells of the present invention can refer toprogenitor cells, i.e., cells having the capacity to develop intovarious types of cells within a specified lineage.

The present invention generally relates to tissue engineering, and moreparticularly to a photocrosslinked biodegradable hydrogel, a method forforming the hydrogel, a method for using the hydrogel in differenttissue engineering applications, a method for attaching heparin tocontrol release of growth factors, and separate methods forincorporating cell adhesion peptides on or within the biodegradablehydrogel. The photocrosslinked biodegradable hydrogel of the presentinvention is substantially cytocompatible (i.e., substantiallynon-cytotoxic) and includes controllable physical properties, such asdegradation rate, swelling behavior, and mechanical properties.

The photocrosslinked biodegradable hydrogel can include a plurality ofnatural polymer macromers cross-linked with a plurality of cross-linksthat are degradable after administration to a subject in vivo. Thenumber or percentage of cross-links linking the macromers can be variedto control the mechanical properties, swelling ratios, and degradationprofiles of the hydrogels. Degradation of the cross-links in vivo allowsthe hydrogel to more readily biodegrade and be used for in vivoapplications. Additionally, as discussed in more detail below, thephotocrosslinked biodegradable hydrogel can be used as a substrate forthe incorporation and/or attachment of various agents and/or cells. Thephotocrosslinked biodegradable hydrogel can be injectable and/orimplantable, and can be in the form of a membrane, sponge, gel, solidscaffold, spun fiber, woven or unwoven mesh, nanoparticle,microparticle, or any other desirable configuration.

In an aspect of the invention, the photocrosslinked biodegradablehydrogel can include at least on cross-link that can be hydrolyzed toallow degradation of the hydrogel in vivo. In one embodiment, thecross-link can include ester, amide, acetal, and/or ketal groups orlinkages that can be readily hydrolyzed in vivo to promote degradationof the hydrogel. In one example, the hydrolyzable cross-link can includeat least one hydrolyzable acrylate (e.g., methacrylate) cross-link. Thehydrolyzable acrylate cross-link can include at least one hydrolyzableester and/or hydrolyzable amide linkage. As explained further below,hydrolytic degradation of the hydrolyzable acrylate cross-link cancreate space for cell growth and deposition of a new extracellularmatrix to replace the photocrosslinked biodegradable hydrogel in vivo.

An example of a biodegradable hydrogel with an acrylate cross-link witha hydrolyzable ester and/or amide linkage has the following Formula I:

where R₁ and R₂ can comprise the same or different natural polymermacromers. The natural polymer macromers can be any natural polymer oroligomer that includes a functional group (e.g., a carboxylic group)that can be further polymerized. Examples of natural polymers oroligomers are saccharides (e.g., mono-, di-, oligo-, andpoly-saccharides), such as glucose, galactose, fructose, lactose andsucrose, collagen, gelatin, glycosaminoglycans, poly(hyaluronic acid),poly(sodium alginate), hyaluronan, alginate, heparin and agarose.

The hydrolyzable acrylate cross-link can be formed by reacting an acrylgroup (e.g., a methacryl group) with a natural polymer or oligomer toform a plurality of acryl substituted macromers. The acryl substitutedmacromers can then be combined with a cross-link initiator, such as aphotoinitiator, and then cross-linked to promote formation of at leastone hydrolyzable acrylate cross-link between the macromers.

In another example of the present invention, the photocrosslinkedbiodegradable hydrogel can have Formula II:

wherein each of R₃ and R₄ can include alginate main chains. As shown inFIG. 1, the photocrosslinked biodegradable hydrogel can comprise aplurality of alginate macromers cross-linked with a plurality ofhydrolyzable methacrylate cross-links. The alginate can be obtained froma commercially available source, such as FMC BIOPOLYMER.

In another example of the present invention, the photocrosslinkedbiodegradable hydrogel can have Formula III:

wherein R₅ can include an alginate main chain; a methacrylated alginatemain chain; and/or an acrylated alginate main chain; and R₆ can includea heparin main chain; a methacrylated heparin main chain; and/or anacrylated heparin main chain. Alternatively, R₆ can comprisepolypeptide, such as a cell adhesion ligand amino acid sequence (e.g., amono-acrylate or mono-methacrylate cell adhesion sequence), a growthfactor (e.g., a mono-acrylate or mono-methacrylate growth factor), adi-acrylate or di-methacrylate cell cleavable amino acid sequence (e.g.,by enzymatic degradation), and the like.

The swelling behavior, mechanical properties, and degradation rates ofthe photocrosslinked biodegradable hydrogel can be controlled bychanging the percentage of cross-links in the hydrogel. The percentageof cross-links can be varied between about 1% and about 50% by weight,and, for example, between about 4% and about 25% by weight. Byincreasing the percentage of cross-links, for example, the degradationrate of the photocrosslinked biodegradable hydrogel can be decreased.Additionally, the compressive stiffness of the photocrosslinkedbiodegradable hydrogel can be increased by increasing the percentage ofcross-links. Further, the swelling behavior of the photocrosslinkedbiodegradable hydrogel can be increased by decreasing the percentage ofcross-links.

In another aspect of the present invention, the photocrosslinkedbiodegradable hydrogel can include at least one cell dispersed on orwithin the hydrogel. For example, cells can be entirely or partlyencapsulated within the photocrosslinked biodegradable hydrogel. Cellscan include any progenitor cell, such as a totipotent stem cell, apluripotent stem cell, or a multipotent stem cell, as well as any oftheir lineage descendant cells, including more differentiated cells(described above), such as CD34⁺MSCs. For example, human MSCs can bephotoencapsulated using a heparin-modified, photocrosslinkedbiodegradable hydrogel (FIG. 2).

The cells can be autologous, xenogeneic, allogeneic, and/or syngeneic.Where the cells are not autologous, it may be desirable to administerimmunosuppressive agents in order to minimize immunorejection. The cellsemployed may be primary cells, expanded cells, or cell lines, and may bedividing or non-dividing cells. Cells may be expanded ex vivo prior tointroduction into or onto the photocrosslinked biodegradable hydrogel.For example, autologous cells can be expanded in this manner if asufficient number of viable cells cannot be harvested from the hostsubject. Alternatively or additionally, the cells may be pieces oftissue, including tissue that has some internal structure. The cells maybe primary tissue explants and preparations thereof, cell lines(including transformed cells), or host cells.

In another aspect of the present invention, the photocrosslinkedbiodegradable hydrogel can include at least one attachment molecule tofacilitate attachment of at least one cell thereto. The attachmentmolecule can include a polypeptide or small molecule, for example, andmay be chemically immobilized onto the photocrosslinked biodegradablehydrogel to facilitate cell attachment. Examples of attachment moleculescan include fibronectin or a portion thereof, collagen or a portionthereof, polypeptides or proteins containing a peptide attachmentsequence (e.g., arginine-glycine-aspartate sequence) (or otherattachment sequence), enzymatically degradable peptide linkages, celladhesion ligands, growth factors, degradable amino acid sequences,and/or protein-sequestering peptide sequences. It will be appreciatedthat the at least one attachment molecule may also improve cellattachment to microspheres, the incorporation of cells into thehydrogel, and hydrogel formation.

In an example of the present invention, an attachment molecule caninclude a peptide having the amino acid sequence of SEQ ID NO: 1 that ischemically immobilized onto the photocrosslinked biodegradable hydrogelto facilitate cell attachment. As shown in FIG. 3, for example, EDC/NHS(carbodiimide) chemistry can be used to prepare methacrylated alginatemodified with at least one attachment molecule having the amino acidsequence of SEQ ID NO: 1. The methacrylated alginate can then becross-linked using a photoinitiator to form a biodegradable hydrogel.

Alternatively, the at least one attachment molecule can be chemicallymodified to include a moiety that can be used to couple the attachmentmolecule to the macromer. For example, at least one attachment moleculehaving the amino acid sequence of SEQ ID NO: 1 can be acrylated (e.g.,methacrylated) as shown in FIG. 4. The acrylated attachment molecule canbe cross-linked with the acrylate macromer to form a cross-linkedbiodegradable hydrogel. The acrylated attachment molecule can also becross-linked with acrylated heparin and acrylated alginate to vary themechanical properties of the hydrogel as shown in FIG. 5.

In another aspect of the present invention, the photocrosslinkedbiodegradable hydrogel can include at least one bioactive agent that iscapable of modulating a function and/or characteristic of a cell. Forexample, the bioactive agent may be capable of modulating a functionand/or characteristic of a cell that is dispersed on or within thephotocrosslinked biodegradable hydrogel. Alternatively or additionally,the bioactive agent may be capable of modulating a function and/orcharacteristic of an endogenous cell surrounding a photocrosslinkedbiodegradable hydrogel implanted in a tissue defect, for example, andguide the cell into the defect.

The at least one bioactive agent can include polynucleotides and/orpolypeptides encoding or comprising, for example, transcription factors,differentiation factors, growth factors, and combinations thereof. Theat least one bioactive agent can also include any agent capable ofpromoting tissue formation (e.g., bone and/or cartilage), destruction,and/or targeting a specific disease state (e.g., cancer). Examples ofbioactive agents include chemotactic agents, various proteins (e.g.,short term peptides, bone morphogenic proteins, collagen, glycoproteins,and lipoprotein), cell attachment mediators, biologically activeligands, integrin binding sequence, various growth and/ordifferentiation agents and fragments thereof (e.g., EGF), HGF, VEGF,fibroblast growth factors (e.g., bFGF), PDGF, insulin-like growth factor(e.g., IGF-I, IGF-II) and transforming growth factors (e.g., TGF-βI-III), parathyroid hormone, parathyroid hormone related peptide, bonemorphogenic proteins (e.g., BMP-2, BMP-4, BMP-6, BMP-7, BMP-12, BMP-13,BMP-14), sonic hedgehog, growth differentiation factors (e.g., GDF5,GDF6, GDF8), recombinant human growth factors (e.g., MP-52 and the MP-52variant rhGDF-5), cartilage-derived morphogenic proteins (CDMP-1,CDMP-2, CDMP-3), small molecules that affect the upregulation ofspecific growth factors, tenascin-C, hyaluronic acid, chondroitinsulfate, fibronectin, decorin, thromboelastin, thrombin-derivedpeptides, heparin-binding domains, heparin, heparin sulfate,polynucleotides, DNA fragments, DNA plasmids, MMPs, TIMPs, interferingRNA molecules, such as siRNAs, DNA encoding for an shRNA of interest,oligonucleotides, proteoglycans, glycoproteins, and glycosaminoglycans.

In another aspect of the present invention, the photocrosslinkedbiodegradable hydrogel can be modified or configured to differentiallyand/or controllably release at least one bioactive agent by forming atleast one concentration gradient within the hydrogel. The hydrogel canhave multiple gradients in the same hydrogel, and the gradients can runin the same or opposite directions. The gradients can be comprised ofdifferent components, such as different photo-alginates having differentmolecular weights or acrylation (e.g., methacrylation) percentages,methacrylated heparin, acrylated cell adhesion ligands, bioactivefactors, cells, etc. As discussed below, for example, thephotocrosslinked biodegradable hydrogel can be formed into a particularshape or form to facilitate release of one or more bioactive agentsaccording to a gradient release profile. Alternatively, one or morematerials or agents can be added to the photocrosslinked biodegradablehydrogel to facilitate differential and/or controlled release of one ormore bioactive agents according to a gradient release profile.

In one example of the present invention, the photocrosslinkedbiodegradable hydrogel can be formed into alginate microbeads ormicrospheres capable of carrying and differentially and/or controllablyreleasing at least one bioactive agent. Alginate microspheres can beprepared by dissolving methacrylated alginate in an about 2% w/vsolution in deionized water containing about 0.05% w/v photoinitiator(e.g., D-2959). About 10 ml of alginate solution can be added to 50 mlof iso-octane containing about 5% v/v Span 80. The resulting solutioncan then be emulsified by sonicating at about 40 W for about 30 seconds.Next, about 2.5 ml of about 30% v/v Tween 80 in water can be added as asurfactant, and the solution stirred for about 5 minutes. To ionicallycrosslink the microspheres, about 10 ml of filtered CaCl₂ (about 700 mM)can be added in a drop-wise fashion and then stirred for about 5minutes. Next, about 50 ml of isopropanol can be added and the particlesstirred for an additional period of time (e.g., about 5 minutes). Theparticles can then be collected by centrifuging at about 4000 RPM forabout 5 minutes, followed by washing about 3 times with about 25 mlisopropanol. The particles can be air dried then resuspended indeionized water containing about 0.05% w/v photoinitiator (e.g.,D-2959). The particles can then be photocrosslinked under UV light forabout 10 minutes (if desired, the calcium can be removed at this pointwith about 1 M citric acid solution for 10 minutes), frozen, andlyophilized.

In another example of the present invention, a heparin-modified,photocrosslinked biodegradable hydrogel can be formed into microbeads ormicrospheres. As described in the example above, alginate microspherescan first be prepared. Next, methacrylated heparin can be added to thealginate solution (e.g., in a concentration of about 10% w/w toalginate). About 10 ml of alginate solution can be added to 50 ml ofiso-octane containing about 5% v/v Span 80. The resulting solution canthen be emulsified by sonicating at about 40 W for about 30 seconds.Next, about 2.5 ml of about 30% v/v Tween 80 in water can be added as asurfactant, and the solution stirred for about 5 minutes. To ionicallycrosslink the microspheres, about 10 ml of filtered CaCl₂ (about 700 mM)can be added in a drop-wise fashion and then stirred for about 5minutes. Next, about 50 ml of isopropanol can be added and the particlesstirred for an additional period of time (e.g., about 5 minutes). Theparticles can then be collected by centrifuging at about 4000 RPM forabout 5 minutes, followed by washing about 3 times with about 25 mlisopropanol. The particles can be air dried then resuspended indeionized water containing about 0.05% w/v photoinitiator (e.g.,D-2959). The particles can then be photocrosslinked under UV light forabout 10 minutes (if desired, the calcium can be removed at this pointwith about 1 M citric acid solution for 10 minutes), frozen, andlyophilized.

As noted above, the concentration gradients can be physically formedwithin the hydrogel to facilitate release of one or more bioactiveagents according to a gradient release profile. The gradient releaseprofile can refer to the amount and/or rate of release of a bioactiveagent from the photocrosslinked degradable hydrogel. The gradientrelease profile can be selected for a particular hydrogel by modifyingat least one property or characteristic (e.g., percentage of acrylationof natural polymers, concentration of attachment molecules, presence ofcarrier material, percentage of cross-links in hydrogel) of thematerial(s) (e.g., type of macromer(s) or carrier material) used to formthe hydrogel. Depending upon the modified property or characteristic, adifferent gradient will be formed and a different release profile willbe produced. During formation of the photocrosslinked biodegradablehydrogel, for example, the concentration of bioactive moleculesincorporated into the hydrogel can be increased or decreased to increaseor decrease the concentration gradient of the bioactive molecules uponrelease from the hydrogel. Examples of gradient release profiles forBMP-2 and TGF-β₁ from alginate hydrogel microbeads are illustrated inFIGS. 6 and 7. It will be appreciated that other techniques can be usedto form hydrogels having at least one gradient, such ascomputer-controlled syringe pumps.

In another example of the present invention, the photocrosslinkedbiodegradable hydrogel can include at least one material or agentdisposed on or within the hydrogel that is capable of carrying anddifferentially and/or controllably releasing at least one bioactiveagent. For instance, the at least one agent or material can include acarrier material that is directly linked to the bioactive agent and/orphysically associated with the bioactive agent. Carrier materials caninclude a variety of known microparticles or nanoparticles including,for example, polymer-based and calcium phosphate-based microparticlesand nanoparticles. It will be appreciated that carrier materials mayalso have similar or identical material compositions as thephotocrosslinked biodegradable hydrogel.

Polymer-based carrier materials can include a biodegradable polymercapable of controllably and/or differentially releasing at least onebioactive agent. For example, a polymer-based carrier material can be abiodegradable polymer in microparticle form. Microparticles can have adiameter less than 1 mm and typically between 1 and 200 microns.Microparticles can include both microspheres and microcapsules, and mayhave an approximately spherical geometry and be of fairly uniform size.Microspheres can have a homogeneous composition, and microcapsules caninclude a core composition (e.g., a bioactive agent) distinct from asurrounding shell. For the purposes of the present invention, the terms“microsphere,” “microparticle,” and “microcapsule” may be usedinterchangeably.

Microparticles can be made with a variety of biocompatible andbiodegradable polymers. Examples of biocompatible, biodegradablepolymers are poly(lactide)s, poly(glycolide)s,poly(lactide-co-glycolide)s, poly(lactic acid)s, poly(glycolic acid)s,poly(lactic acid-co-glycolic acid)s, polycaprolactone, polycarbonates,polyesteramides, polyanhydrides, poly(amino acids), polyorthoesters,polyacetyls, polycyanoacrylates, polyetheresters, poly(dioxanone)s,poly(alkylene alkylate)s, copolymers of polyethylene glycol andpoly(lactide)s or poly(lactide-co-glycolide)s, biodegradablepolyurethanes, and blends and/or copolymers thereof.

Other examples of materials that may be used to form microparticles caninclude chitosan, poly(ethylene oxide), poly(lactic acid), poly(acrylicacid), poly(vinyl alcohol), poly(urethane), poly(N-isopropylacrylamide), poly(vinyl pyrrolidone) (PVP), poly(methacrylic acid),poly(p-styrene carboxylic acid), poly(p-styrenesulfonic acid),poly(vinylsulfonicacid), poly(ethyleneimine), poly(vinylamine),poly(anhydride), poly(L-lysine), poly(L-glutamic acid),poly(gamma-glutamic acid), poly(carprolactone), polylactide,poly(ethylene), poly(propylene), poly(glycolide),poly(lactide-co-glycolide), poly(amide), poly(hydroxylacid),poly(sulfone), poly(amine), poly(saccharide), poly(HEMA),poly(anhydride), collagen, fibrin, gelatin, glycosaminoglycans (GAG),poly(hyaluronic acid), poly(sodium alginate), alginate, hyaluronan,agarose, polyhydroxybutyrate (PHB), copolymers thereof, and blendsthereof.

In one example of the present invention, a carrier material can comprisea microparticle made of poly(d,l-lactide-co-glycolide) (PLGA). PLGAdegrades when exposed to physiological pH and hydrolyzes to form lacticacid and glycolic acid, which are normal byproducts of cellularmetabolism. The disintegration rate of PLGA polymers may vary dependingon the polymer molecular weight, ratio of lactide to glycolide monomersin the polymer chain, and stereoregularity of the monomer subunits. Forexample, mixtures of L and D stereoisomers that disrupt the polymercrystallinity can increase polymer disintegration rates. In addition, itwill be appreciated that microspheres may contain blends of two or morebiodegradable polymers of different molecular weight and/or monomerratio.

Carrier materials can alternatively comprise a nanoparticle, such assubmicron particles, for controlled release of the bioactive agent. Ananoparticle can have a diameter ranging from about less than 1nanometer to about 1 micron. Nanoparticles can be created in the samemanner as microparticles, except that high-speed mixing orhomogenization may be used to reduce the size of thenanoparticle/bioactive agent emulsion(s) to less than about 2 microns.Alternative methods for nanoparticle production are known in the art andmay be employed for the present invention.

In another example of the present invention, at least one bioactiveagent can be incorporated on or within at least one calcium phosphatenanoparticle dispersed on or within the photocrosslinked biodegradablehydrogel. The at least one calcium phosphate nanoparticle candifferentially or controllably release the at least one bioactive agentor be taken up (e.g., via endocytosis) by at least one cell to modulatethe function and/or characteristic of the cell. The at least onebioactive agent may be at least partially coated on the surface of atleast one calcium phosphate nanoparticle. Alternatively, the at leastone bioactive agent may be dispersed, incorporated, and/or impregnatedwithin the calcium phosphate nanoparticle. For example, a bioactiveagent comprising a DNA plasmid (e.g., a plasmid encoding BMP-2) can becoated onto the surface of the calcium phosphate nanoparticle.Alternatively, a DNA plasmid can be co-precipitated with calciumphosphate to form the calcium phosphate nanoparticle. After forming thecalcium phosphate nanoparticles, the nanoparticles can be coated withDNA or protein to prevent nanoparticle aggregation and/or promotecellular uptake. It will be appreciated that one or more of the same ordifferent bioactive agents can be incorporated on or within the at leastone calcium phosphate nanoparticle.

Calcium phosphate nanoparticles can have an average particle size ofbetween about 1 nm and about 200 nm. It will be appreciated that smalleror larger calcium phosphate nanoparticles may be used. The calciumphosphate nanoparticles can have a generally spherical morphology and beof a substantially uniform size or, alternatively, may be irregular inmorphology. Calcium phosphate nanoparticles may be complexed withsurface modifying agents to provide a threshold surface energysufficient to bind material (e.g., bioactive agents) to the surface ofthe nanoparticle without denaturing the material. Non-limiting examplesof surface modifying agents can include basic or modified sugars, suchas cellobiose, carbohydrates, carbohydrate derivatives, macromoleculeswith carbohydrate-like components characterized by an abundance of —OHside groups and polyethylene glycol.

In another example of the present invention, a bioactive agent cancomprise an interfering RNA molecule incorporated on or within at leastone carrier material dispersed on or within the photocrosslinkedbiodegradable hydrogel. The interfering RNA molecule can include any RNAmolecule that is capable of silencing a target mRNA and thereby reducingor inhibiting expression of a polypeptide encoded by the target mRNA.Alternatively, the interfering RNA molecule can include a DNA moleculeencoding for a shRNA of interest. For example, the interfering RNAmolecule can comprise a short interfering RNA (siRNA) or microRNAmolecule capable of silencing a target mRNA that encodes any one orcombination of the polypeptides or proteins described above. The atleast one carrier material can differentially or controllably releasethe at least one interfering RNA molecule or be taken up (e.g., viaendocytosis) by at least one cell to modulate a function and/orcharacteristic of the cell.

In another aspect of the present invention, the photocrosslinkedbiodegradable hydrogel can include first and second bioactive agentsdisposed on or within the hydrogel. The first and second bioactiveagents may comprise the same or different agents. As described infurther detail below, the first and second bioactive agents can bedifferentially, sequentially, and/or controllably released from thephotocrosslinked biodegradable hydrogel to modulate a different functionand/or characteristic of a cell. It will be appreciated that the firstbioactive agent can have a release profile that is the same or differentfrom the release profile of the second bioactive agent from thephotocrosslinked biodegradable hydrogel. The first and second bioactiveagents may be dispersed uniformly on or within the photocrosslinkedbiodegradable hydrogel or, alternatively, dispersed such that differentdensities of the bioactive agents are localized on or within differentportions of the hydrogel. It should also be appreciated that themacromer scaffold can be in either a hydrated or lyophilized state priorto the addition of bioactive agents. For example, the macromer scaffoldcan be in a lyophilized state before the addition of bioactive agents isdone to re-hydrate and populate the scaffold with bioactive agents.

Alternatively, at least one bioactive agent can be chemically modifiedto include a moiety that can be used to couple the at least onebioactive agent to the macromer. For example, at least one bioactiveagent can be acrylated (e.g., methacrylated). The acrylated bioactiveagent can be cross-linked with the acrylate macromer to form across-linked biodegradable hydrogel. The acrylated bioactive agent canalso be cross-linked with acrylated heparin and acrylated alginate tovary the mechanical properties of the hydrogel.

In another aspect of the present invention, the photocrosslinkedbiodegradable hydrogel can include first and second bioactive agentsrespectively incorporated on or within first and second carriermaterials. The first and second carrier materials may comprise the sameor different materials. Additionally, the first and second bioactiveagents may comprise the same or different agents. As described infurther detail below, the first and second carrier materials candifferentially, sequentially, and/or controllably release the first andsecond bioactive agents to modulate a different function and/orcharacteristic of a cell. It will be appreciated that the first carriermaterial can release the first bioactive agent with a different releaseprofile than the release profile of the second bioactive agent from thesecond carrier material. Additionally, it will be appreciated that thefirst carrier material can degrade or diffuse before the degradation ordiffusion of the second carrier material or allow for an increased rateof release or diffusion of the first bioactive agent compared to therelease of the second bioactive agent. The first and second carriermaterials may be dispersed uniformly on or within the photocrosslinkedbiodegradable hydrogel or, alternatively, dispersed such that differentdensities of carrier materials are localized on or within differentportions of the hydrogel.

Another aspect of the invention relates to a method of forming aphotocrosslinked biodegradable hydrogel that is capable of serving as asubstrate for the incorporation and/or attachment of at least onebioactive agent and at least one cell. The method can include reactingat least one acryl group with at least one natural polymer macromer toform a mixture comprising a plurality of acrylated macromers. Themixture can be contacted or combined with a photoinitiator that caninitiate cross-linking of the macromers. The mixture can be exposed to alight source to initiate cross-linking of the macromers and thereby forma plurality of natural polymer macromers cross-linked with a pluralityof hydrolyzable acrylate cross-links.

FIG. 1 illustrates an example of a method of forming a photocrosslinkedbiodegradable hydrogel comprising a plurality of methacrylated alginatemacromers cross-linked with a plurality of hydrolyzable methacrylatecross-links. The method includes preparing methacrylated alginatemacromers by reacting low molecular weight alginate with 2-aminoethylmethacrylated (AEMA). Low molecular weight sodium alginate can beprepared by irradiating the alginate at a gamma dose of about 5 Mrad/hrfor about 4 hours. The low molecular weight sodium alginate can then bedissolved in a buffer solution (about 1 w/v % at a pH of about 6.5) ofabout 50 mM 2-morpholinoethanesulfonic acid (MES) containing about 0.5 MNaCl. N-hydroxysuccinimide (NHS) and1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide hydrochloride (EDC) (atan NHS:EDC ratio of about 1:2) can be added to the solution to activatethe carboxylic acid groups of the alginate. After about 5 minutes, AEMAcan be added to the solution. The reaction can be maintained at roomtemperature for about 24 hours. After the reaction, the mixture can beprecipitated by pouring into an excess of acetone, drying under reducedpressure, and rehydrating to about a 1% w/v solution in ultrapuredeionized water (diH₂O) for further purification. The methacrylatedalginate can be purified by dialysis in diH₂O using a dialysis membranefor 3 days, followed by filtering through about a 0.22 μm filter, andthen lyophilization.

The methacrylated alginate can then be modified with at least oneattachment molecule. For example, at least one attachment moleculehaving the amino acid sequence of SEQ ID NO: 1 can be reacted with themethacrylated alginate about 5 minutes after forming the methacrylatedalginate. The reaction mixture can then be maintained at roomtemperature for about 24 hours. Next, the modified alginate can bepurified with dialysis in diH₂O for about 3 days. After dialysis, about0.5 g of activated carbon can be added for about every 1 g alginate. Thesolution can then be lyophilized after removing the activated carbon byfiltration.

After forming the methacrylated alginate, either with or withoutattachment molecules, the methacrylated alginate can be photocrosslinkedby dissolving a desired amount of the alginate in an appropriate amountof diH₂O or aqueous media (e.g., PBS). A desired amount of aphotoinitiator (e.g., Irgacure D2959) can then be added to the solution.The solution can then be injected into a curing vessel (e.g., two glassplates separated by spacers) and exposed to a light source at awavelength and for a time to promote cross-linking of the methacrylatedalginate macromers and form the photocrosslinked biodegradable hydrogel.For example, the methacrylated alginate can be exposed to UV light ofabout 365 nm at about 8 to 20 mW/cm² for about 8 minutes to form thehydrogel.

In another example of the present invention, the method can includeforming a heparin-modified, photocrosslinked biodegradable hydrogel. Asshown in FIG. 8, the method can include immersing heparin in MES. Theheparin can be dissolved in a buffer solution (about 10 w/v % at aboutpH 6.5) of about 50 mM MES containing about 0.5 M NaCl. NHS and EDC (atan NHS:EDC ratio of about 1:2) can be added to the solution to activatethe carboxylic acid groups of heparin. After about 5 minutes, AEMA canbe added to the solution. The reaction may be maintained at about roomtemperature for about 24 hours. After the reaction, the mixture can beprecipitated by pouring into an excess of acetone, drying under avacuum, and rehydrating to about a 10% w/v solution in diH₂O for furtherpurification. Methacrylated alginate can then be purified by dialysis indiH₂O using a dialysis membrane for about 3 days, filtered through abouta 0.22 μm filter, and the lyophilized until dry. It will be appreciatedthat heparin can be used in varying concentration to control the releaserate of a bioactive agent or agents.

After forming the heparin-modified, methacrylated alginate macromers,either with or without attachment molecules, the macromers can bephotocrosslinked by dissolving a desired amount of the macromers in anappropriate amount of diH₂O. A desired amount of a photoinitiator (e.g.,Irgacure D2959) can then be added to the solution. The solution can thenbe injected into a curing vessel (e.g., two glass plates separated byspacers) and exposed to a light source at a wavelength and for a time topromote cross-linking of the heparin-modified methacrylated alginatemacromers and form the photocrosslinked biodegradable hydrogel. Forexample, the heparin-modified methacrylated alginate macromers can beexposed to UV light of about 365 nm at about 8 to 20 mW/cm² for about 8minutes to form the hydrogel.

It will be appreciated that the method can include the additional stepof electrospinning acrylated monomers with or without a carrier material(e.g., polyethylene oxide or PEO) (FIG. 9). For example, Table 1illustrates electrospinning conditions of photocrosslinked alginate/PEOnanofibrous scaffolds.

TABLE 1 Electrospinning Conditions of Photocrosslinked Alginate/PEONanofibrous Scaffolds Electrospinning conditions MA-pure- PEO Tip-to-And RGD-alginate^(a) Concentration^(a) Needle collector Concentration(wt %) Alginate:PEO Voltage size distance Flow rate Sample code (wt %)(Mv = 900,000) (vol/vol %) (kV) (G) (cm) (ml/min) 1PMAlg/4PEO 1.0 4.050:50 (0.5:2.0) 9.4-10.4 20 15 0.2 0.05 2PMAlg/4PEO 2.0 50:50 (1.0:2.0)4PMAlg/4PEO 4.0 50:50 (2.0:2.0) 8PMAlg/4PEO 8.0 50:50 (4.0:2.0)8RMAlg/4PEO 8.0 4.0 50:50 (4.0:2.0) 8HRMAlg/4PEO 8.0 4.0 50:50 (4.0:2.0)^(a)Methacrylated alginate and PEO were dissolved in diH₂O (10 ml) with0.05 w/v photoinitiator.

It should also be appreciated that the photocrosslinked biodegradablehydrogel can be formed with at least one cell and/or bioactive agent.For example, a plurality of cells may be dispersed in a substantiallyuniform manner on or within the hydrogel or, alternatively, dispersedsuch that different densities and/or spatial distributions of differentor the same cells are dispersed within different portions of thehydrogel. The cells can be autologous, allogeneic or xenogeneic. It willalso be appreciated that the cells may be seeded before or after themacromers are photocrosslinked. Alternatively, photocrosslinkedhydrogels can be incubated in a solution of at least one bioactive agentafter the macromers are cross-linked.

Generally, cells can be introduced into the photocrosslinkedbiodegradable hydrogel in vitro, although in vivo seeding approaches canoptionally or additionally be employed. Cells may be mixed with thephotocrosslinked biodegradable hydrogel and cultured in an adequategrowth (or storage) medium to ensure cell viability. If thephotocrosslinked biodegradable hydrogel is to be implanted for use invivo after in vitro seeding, for example, sufficient growth medium maybe supplied to ensure cell viability during in vitro culture prior to invivo application. Once the photocrosslinked biodegradable hydrogel hasbeen implanted, the nutritional requirements of the cells can be met bythe circulating fluids of the host subject.

Any available method may be employed to introduce the cells into thephotocrosslinked biodegradable hydrogel. For example, cells may beinjected into the photocrosslinked biodegradable hydrogel (e.g., incombination with growth medium) or may be introduced by other means,such as pressure, vacuum, osmosis, or manual mixing. Alternatively oradditionally, cells may be layered on the photocrosslinked biodegradablehydrogel, or the hydrogel may be dipped into a cell suspension andallowed to remain there under conditions and for a time sufficient forthe cells to incorporate within or attach to the hydrogel. Generally, itis desirable to avoid excessive manual manipulation of the cells inorder to minimize cell death during the impregnation procedure. Forexample, in some situations it may not be desirable to manually mix orknead the cells with the photocrosslinked biodegradable hydrogel;however, such an approach may be useful in those cases in which asufficient number of cells will survive the procedure. Cells can also beintroduced into the photocrosslinked biodegradable hydrogel in vivosimply by placing the hydrogel in the subject adjacent a source ofdesired cells. Bioactive agents released from the photocrosslinkedbiodegradable hydrogel may also recruit local cells, cells in thecirculation, or cells at a distance from the implantation or injectionsite.

As those of ordinary skill in the art will appreciate, the number ofcells to be introduced into the photocrosslinked biodegradable hydrogelwill vary based on the intended application of the hydrogel and on thetype of cell used. Where dividing autologous cells are being introducedby injection or mixing into the photocrosslinked biodegradable hydrogel,for example, a lower number of cells can be used. Alternatively, wherenon-dividing cells are being introduced by injection or mixing into thephotocrosslinked biodegradable hydrogel, a larger number of cells may berequired. It should also be appreciated that the macromer scaffold canbe in either a hydrated or lyophilized state prior to the addition ofcells. For example, the macromer scaffold can be in a lyophilized statebefore the addition of cells is done to re-hydrate and populate thescaffold with cells.

In an example of the present invention, a plurality of chondrocytes canbe seeded on onto the photocrosslinked biodegradable hydrogel at adesired density and allowed to adhere thereto for a period of time. Thechondrocytes can be obtained from a host subject and then expanded exvivo to a desired density. For example, about 1×10⁴ cells/cm² can beseeded onto a photocrosslinked biodegradable hydrogel and then allowedto adhere thereto for about 4 hours. Alternatively, at least one cellcan be partly or entirely encapsulated in the photocrosslinkedbiodegradable hydrogel. To encapsulate at least one chondrocyte, forexample, a population of chondrocytes can be suspended a methacrylatedalginate solution (about 2 w/v % in DMEM) with about 0.05 w/v %photoinitiator. The cell/macromer solutions (about 300 μl) can then bepipetted into a vessel (e.g., a 96-well culture plate) at a desireddensity (e.g., 1×10⁷ cells/ml) and photocrosslinked with UV for about 10minutes. The resulting cell-seeded hydrogel can be placed in culture orused for a desired tissue engineering application, for example.

To form a photocrosslinked biodegradable hydrogel with at least onebioactive agent, a methacrylated alginate solution can be completelydissolved in diH₂O or other aqueous media and then mixed with a desiredamount of a photoinitiator. At least one growth factor (e.g., TGF-β1,bFGF, VEGF or BMP-2) can then be added to the solution at a desiredconcentration (e.g., about 0.5 μg/ml). Aliquots of the solution can thenbe placed in a container (e.g., a 96-well plate) and photocrosslinkedwith about 365 nm UV light at about 8 to 20 mW/cm² for about 8 minutes.

The photocrosslinked biodegradable hydrogel can be used in a variety ofbiomedical applications, including tissue engineering, drug discoveryapplications, and regenerative medicine. In one example of the presentinvention, a photocrosslinked biodegradable hydrogel can be used topromote tissue growth in a subject. One step of the method can includeidentifying a target site. The target site can comprise a tissue defect(e.g., cartilage and/or bone defect) in which promotion of new tissue(e.g., cartilage and/or bone) is desired. The target site can alsocomprise a diseased location (e.g., tumor). Methods for identifyingtissue defects and disease locations are known in the art and caninclude, for example, various imaging modalities, such as CT, MRI, andX-ray.

The tissue defect can include a defect caused by the destruction of boneor cartilage. For example, one type of cartilage defect can include ajoint surface defect. Joint surface defects can be the result of aphysical injury to one or more joints or, alternatively, a result ofgenetic or environmental factors. Most frequently, but not exclusively,such a defect will occur in the knee and will be caused by trauma,ligamentous instability, malalignment of the extremity, meniscectomy,failed aci or mosaicplasty procedures, primary osteochondritisdessecans, osteoarthritis (early osteoarthritis or unicompartimentalosteochondral defects), or tissue removal (e.g., due to cancer).Examples of bone defects can include any structural and/or functionalskeletal abnormalities. Non-limiting examples of bone defects caninclude those associated with vertebral body or disc injury/destruction,spinal fusion, injured meniscus, avascular necrosis, cranio-facialrepair/reconstruction (including dental repair/reconstruction),osteoarthritis, osteosclerosis, osteoporosis, implant fixation, trauma,and other inheritable or acquired bone disorders and diseases.

Tissue defects can also include cartilage defects. Where a tissue defectcomprises a cartilage defect, the cartilage defect may also be referredto as an osteochondral defect when there is damage to articularcartilage and underlying (subchondral) bone. Usually, osteochondraldefects appear on specific weight-bearing spots at the ends of thethighbone, shinbone, and the back of the kneecap. Cartilage defects inthe context of the present invention should also be understood tocomprise those conditions where surgical repair of cartilage isrequired, such as cosmetic surgery (e.g., nose, ear). Thus, cartilagedefects can occur anywhere in the body where cartilage formation isdisrupted, where cartilage is damaged or non-existent due to a geneticdefect, where cartilage is important for the structure or functioning ofan organ (e.g., structures such as menisci, the ear, the nose, thelarynx, the trachea, the bronchi, structures of the heart valves, partof the costae, synchondroses, enthuses, etc.), and/or where cartilage isremoved due to cancer, for example.

After identifying a target site, such as a cranio-facial cartilagedefect of the nose, the photocrosslinked biodegradable hydrogel can beadministered to the target site. The hydrogel can be prepared accordingto the method described above. For example, a plurality of cells, suchas chondrocytes may be mixed with a plurality of methacrylated alginatemacromers to form a solution. The solution may also be mixed with atleast one attachment molecule, such as a an acrylated or methacrylatedpolypeptide having the amino acid sequence of SEQ ID NO: 1. For example,this step can be carried out if the at least one attachment molecule ismixed with an acrylated or methacrylated solution prior to crosslinking.Otherwise, the at least one attachment molecule can be previously boundto the alginate macromers using carbodiimide chemistry, for example.Chondrocytes may be obtained from a host subject and then expanded to adesired density ex vivo.

Next, the biodegradable hydrogel may be loaded into a syringe or othersimilar device and injected or implanted into the tissue defect. Uponinjection or implantation into the tissue defect, the biodegradablehydrogel be formed into the shape of the tissue defect using tactilemeans. Alternatively, the biodegradable may be formed into a specificshape prior to injection or implantation into the subject. A transdermallight source (e.g., a UV light source) can then be applied to the areaof the subject's skin substantially adjacent the tissue defect topromote photocrosslinking of the methacrylated alginate macromers andformation of a photocrosslinked biodegradable hydrogel havingsubstantially identical dimensions of the tissue defect. Alternatively,it will be appreciated that the light source can be directly applied tothe hydrogel where the hydrogel is first placed in an open wound ordefect.

After implanting the photocrosslinked biodegradable hydrogel into thesubject, the chondrocytes can begin to migrate from the hydrogel intothe tissue defect, express growth and/or differentiation factors, and/orpromote chondroprogenitor cell expansion and differentiation.Additionally, the presence of the photocrosslinked biodegradablehydrogel in the tissue defect may promote migration of endogenous cellssurrounding the tissue defect into the photocrosslinked biodegradablehydrogel. Once implanted, the amide and/or ester linkages of thehydrolyzable methacrylated cross-links can be hydrolyzed. Hydrolysis ofthe methacrylated cross-links can occur at a controlled rate and lead tocontrolled degradation of the photocrosslinked biodegradable hydrogel.This hydrolytic degradation can create space for cell growth anddeposition of a new extracellular matrix to replace the hydrogel.

In another example of the present invention, a photocrosslinkedbiodegradable hydrogel can be prepared according to the method describedabove. For example, a photocrosslinked biodegradable hydrogel cancomprise: a plurality of polypeptide-modified, methacrylated alginatemacromers cross-linked with a plurality of hydrolyzable methacrylatecross-links; at least one CD34⁺MSC; and first and second carriermaterials respectively including first and second bioactive agents. Themethacrylated alginate macromers can be modified with at least onepolypeptide having the amino acid sequence of SEQ ID NO: 1. Each of thefirst and second carrier materials can be comprised of PLGA and may beprepared as described above. For example, the first carrier material maycomprise a greater mixture of L and D stereoisomers to increase thedegradation rate of the first carrier material. Additionally, the secondcarrier material may comprise a lower mixture of L and D stereoisomers(as compared to the first carrier material) so that the second carriermaterial has a slower degradation rate when exposed to physiologicalconditions.

The first and second bioactive agents may then be impregnated intoand/or coated onto the first and second carrier materials, respectively.The first bioactive agent can comprise a growth factor (e.g., TGF-β,VEGF and/or FGF-2) or, alternatively, a plasmid including apolynucleotide that encodes a growth factor (e.g., TGF-β, VEGF and/orFGF-2). Similarly, the second bioactive agent can comprise a growthfactor (e.g., IGF-I and/or BMP-2) or, alternatively, a plasmid includinga polynucleotide that encodes a growth factor (e.g., IGF-I and/orBMP-2). It should be appreciated that in order to regenerate bone, FGF-2and/or VEGF can be incorporated into the first bioactive agent and BMP-2(or a DNA plasmid encoding BMP-2) can be incorporated into first andsecond carrier materials (e.g., made of PLGA).

Next, at least one CD34⁺MSC may be obtained from a source (e.g., thebone marrow of the host subject) and then expanded ex vivo (e.g., usingFGF-2) to a desired density. After expanding the cells to a desireddensity, the cells can be mixed with the methacrylated alginatemacromers to form a mixture. The biodegradable hydrogel may then beloaded into a syringe or other similar device and injected or implantedinto the tissue defect. A transdermal light source (e.g., a UV lightsource) can then be applied to the area of the subject's skinsubstantially adjacent the tissue defect to promote photocrosslinking ofthe methacrylated alginate macromers and formation of a photocrosslinkedbiodegradable hydrogel having substantially identical dimensions of thetissue defect. Alternatively, it will be appreciated that the lightsource can be directly applied to the hydrogel where the hydrogel isfirst placed in an open wound or defect.

After implanting the photocrosslinked biodegradable hydrogel in thesubject, the first carrier material may begin to degrade faster than thesecond carrier material (or allow for increased diffusion relative tothe first carrier material) and thereby release the growth factor (e.g.,TGF-β) or the polynucleotide encoding the growth factor. Release ofTGF-β from the first carrier material can promote early CD34⁺ MSCcommitment to a particular lineage (e.g., chondrogenic lineage). As thecells proliferate, the second carrier material may degrade more slowlythan the first carrier material and thereby release the other growthfactor (e.g., IGF-I) or the polynucleotide encoding the other growthfactor at a slower rate. Release of IGF-I from the second carriermaterial can promote differentiation of the cells into more mature cells(e.g., chondroprogenitor cells). The continued release of IGF-I, alongwith other growth and/or differentiation factors expressed by the cells(i.e., the cells comprising the hydrogel as well as the cellssurrounding the tissue defect), can promote development of mature cells(e.g., chondrocytes) capable of generating new tissue (e.g., cartilage)for repair of the tissue defect. Additionally, the amide and/or esterlinkages of the hydrolyzable methacrylated cross-links can be hydrolyzedupon implantation of the photocrosslinked biodegradable hydrogel.Hydrolysis of the methacrylated cross-links can occur at a controlledrate and lead to controlled degradation of the photocrosslinkedbiodegradable hydrogel. This hydrolytic degradation can create space forcell growth and deposition of a new extracellular matrix to replace thephotocrosslinked biodegradable hydrogel.

In another example of the present invention, a method is provided forforming a high density cell aggregate. The high density cell aggregatecan comprise a population of cells, a plurality of photocrosslinkedbiodegradable hydrogels, and at least one bioactive agent incorporatedon or within the hydrogels. The photocrosslinked biodegradable hydrogelscan be formed into nanoparticles and/or microparticle, and the cells caninclude undifferentiated progenitor cells, substantially differentiatedprogenitor cells, and/or differentiated cells.

One step of the method can include forming a plurality ofphotocrosslinked biodegradable hydrogels into nanoparticles and/ormicroparticles. The hydrogels can be formed as described above, forexample, and include a plurality of cross-linked methacrylated alginatemacromers. After forming the plurality of photocrosslinked biodegradablehydrogels, the hydrogels can be loaded with at least one bioactiveagent. Methods for forming nanoparticles and/or microparticles loadedwith bioactive agents are known in the art.

After loading the photocrosslinked biodegradable hydrogels, thehydrogels can be mixed with a population of cells to form a cellaggregate. The cells can include any totipotent stem cell, pluripotentstem cell, or multipotent stem cell, and/or differentiated cell.Progenitor cells can include autologous cells; however, it will beappreciated that xenogeneic, allogeneic, or syngeneic cells may also beused. The progenitor cells employed may be primary cells, expanded cellsor cell lines, and may be dividing or non-dividing cells. The cells canbe derived from any desired source. For example, the cells may bederived from primary tissue explants and preparations thereof, celllines (including transformed cells) that have been passaged once (P1),twice (P2), or even more times, or host cells (e.g., human hosts). Anyknown method may be employed to harvest cells for use in the presentinvention. For example, MSCs, which can differentiate into a variety ofmesenchymal or connective tissues (e.g., adipose tissue, osseous tissue,cartilaginous tissue, elastic tissue, and fibrous connective tissues),can be isolated according to the techniques disclosed in U.S. Pat. No.5,486,359 to Caplan et al. and U.S. Pat. No. 5,226,914 to Caplan et al.,the entireties of which are hereby incorporated by reference. In oneexample of the present invention, the population of cells can comprise apopulation of human CD 34⁺ MSCs.

Once the cells and the photocrosslinked biodegradable hydrogels havebeen mixed, the cell aggregate can be centrifuged at a desiredacceleration and for a desired period of time to form the high densitycell aggregate. The photocrosslinked biodegradable hydrogels may bedispersed throughout the aggregate in a substantially uniform manner.The bioactive agent can then be released from the photocrosslinkedbiodegradable hydrogels via diffusion and/or as the hydrogels begin todegrade. Controlled release of the bioactive agent from the particlesmay be dependent on the size and composition of the photocrosslinkedbiodegradable hydrogels, as well as the composition of the medium inwhich the aggregate is immersed. For example, the release rate of thebioactive agent(s) can be selectively controlled by changing the degreeor percent of macromer methacrylation and/or the macromer concentration.

The high density cell aggregate can allow for substantially more uniformspatial delivery of the bioactive agent throughout the interior of theaggregate. The substantially uniform distribution of thephotocrosslinked biodegradable hydrogels and relatively uniform releaseof the bioactive agent in the high density cell aggregate isadvantageous for several reasons, including, but not limited to: (1)rapidly inducing uniform cell differentiation; (2) reducing oreliminating in vitro culture of aggregates prior to utilization in invivo regeneration strategies; (3) providing control over the temporalpresentation of growth factors; and (4) allowing for the use of lowerconcentrations of growth factors as compared to systems employingexogenously-supplied growth factors.

It will be appreciated that the high density cell aggregate can furtherinclude additional photocrosslinked biodegradable hydrogels that includesecond bioactive agents. The second bioactive agents may be the same ordifferent type of agent (described above). The photocrosslinkedbiodegradable hydrogels can differentially, sequentially, and/orcontrollably release the different bioactive agents to modulate the sameor different function and/or characteristics of at least one cell in theaggregate. The bioactive agents can have the same or different releaseprofiles from the photocrosslinked biodegradable hydrogels.

The following example is for the purpose of illustration only and is notintended to limit the scope of the claims, which are appended hereto.

Example 1 Synthesis of Methacrylated Alginate

The methacrylated alginate was prepared by reacting low molecular weightalginate with 2-aminoethyl methacrylate (AEMA, Sigma, St. Louis, Mo.,USA). Low molecular weight of sodium alginate was prepared byirradiating Protanal LF 20/40 (FMC Biopolymer, Philadelphia, Pa., USA)at a gamma dose of 5 Mrad/hr for 4 hours. Low molecular weight sodiumalginate was dissolved in a buffer solution (1 w/v %, pH 6.5) of 50 mM2-morpholinoethanesulfonic acid (MES, Sigma) containing 0.5 M NaCl.N-hydroxysuccinimide (NHS, Sigma) and1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide hydrochloride (EDC,Sigma) (NHS:EDC=1:2) were added to the solution to activate thecarboxylic acid groups of the alginate. After 5 min, AEMA was added tothe solution. The reaction was maintained at room temperature for 24hours. After the reaction, the mixture was precipitated by pouring intoan excess of acetone, dried under reduced pressure, and rehydrated to a1% w/v solution in ultrapure deionized water (diH₂O) for furtherpurification. The methacrylated alginate was purified by dialysis indiH₂O using dialysis membrane (MWCO 3500; Spectrum Laboratories Inc.,Rancho Dominguez, Calif., USA) for 3 days, filtered through a 0.22 μmfilter, and lyophilized.

Photocrosslinking

To fabricate photocrosslinkable alginate hydrogel, methacrylatedalginate (0.2 g) was dissolved in diH₂O (10 ml) with 0.05 w/v %photoinitiator (Irgacure D-2959, Sigma). The alginate solution wasinjected between two glass plates separated by 0.75 mm spacers andphotocrosslinked with 365 nm UV light at ˜8-20 mW/cm² for 8 min to formthe hydrogel. Photocrosslinked hydrogel disks were punched out using a 6mm diameter biopsy punch and placed in the diH₂O.

¹H-NMR Characterization

Methacrylated alginate was dissolved in deuterium oxide with 0.05 w/v %photoinitiator, placed in NMR tube, and photocrosslinked at 365 nm UVlight at ˜8-20 mW/cm² for 8 min to form a hydrogel. The ¹H-NMR spectraof the photocrosslinked alginate hydrogel were recorded using a VarianUnity-300 (300 MHz) NMR spectrometer (Varian Inc., Palo Alto, Calif.,USA). To analyze the methacrylation efficiency of alginate, ¹H-NMRspectra were also recorded for uncrosslinked, unmodified alginate anduncrosslinked, methacrylated alginate. The efficiency of alginatemethacrylation was calculated from ¹H-NMR spectra based on the ratio ofthe integrals for the alginate protons and the methylene protons ofmethacrylate.

Swelling and In Vitro Degradation of Photocrosslinked Alginate Hydrogel

The photocrosslinked alginate hydrogels were lyophilized until dry anddry weights (W_(d)) were measured. Dried hydrogel samples (n=3) wereimmersed in 50 ml of diH₂O at 37° C. and allowed to swell. Swollenhydrogel samples were weighted as W_(s) at different time points. Theswelling ratio (Q) was calculated by Q=W_(s)/W_(d).

The dried hydrogel samples (n=3) were weighed (W_(i)) and incubated in50 ml of diH₂O at 37° C. for in vitro degradation test. The diH₂O wasreplaced every three days. At predetermined time points, the sampleswere removed, rinsed with diH₂O, lyophilized and weighed (W_(d)). Thepercent mass loss was calculated by (W_(i)−W_(d))/W_(i)×100.

Mechanical Testing

The elastic moduli of the photocrosslinked alginate hydrogels wasdetermined by performing constant strain rate compression tests usingthe Rheometrics Solid Analyzer (RSAII, Rheometrics Inc., Piscataway,N.J., USA). The photocrosslinked alginate hydrogel disks were preparedas described in section 2.2., and kept in diH₂O at 37° C. Atpredetermined time points, swollen alginate hydrogel disks were punchedonce again 6 mm diameter disks, and uniaxial unconfined compressiontests were performed on the swollen alginate hydrogels at roomtemperature using a constant crosshead speed of 1.0 cm/min and a loadcell of 10N. Each compressive test was performed for less than 2 secondsto avoid loss of water during measurement. Compressive moduli ofphotocrosslinked alginate hydrogels were obtained from the slope ofstress vs. strain curves, limited to the first 2% of strain.

The number average molecular weight between crosslinks (M_(c)) wascalculated according to following equation; M_(c)=3ρRT/E (Jeon, O. etal., Carbohyd Polym. 70(3):251-257, 2007; Ravi, N. et al., Polymer47(11):4203-4209, 2006). T is the temperature (298 K) at which themodulus was measured, ρ is the concentration of alginate (g/m³) in thecross-linking solution, R is the gas constant (8.3145 Jmol⁻¹K⁻¹), and Eis the compressive modulus.

Cytotoxicity of Methacrylated Alginate and Photocrosslinked AlginateHydrogel

To evaluate cytotoxicities of methacrylated alginate andphotocrosslinked alginate hydrogels, an indirect contact methodology wasemployed. Briefly, MC3T3-E1 Subclone 4 cells (ATCC CRL-2593, Manassas,Va., USA), a mouse pre-osteoblast cell line, were plated in 6-wellplates at 1×10⁶ cells/well in 3 ml of Dulbecco's modified eagle medium(DMEM) and cultured at 37° C., 95% humidity and 5% CO₂ for 24 h. Cellculture inserts (25 mm in diameter, 8 μm pore size on PET track-etchedmembrane; Becton Dickinson Labware Europe, Le Pont De Claix, France)were placed into each well. Sterile methacrylated alginate solution (1ml, 2 w/v % in ultrapure deinonized water) or a photocrosslinkedalginate hydrogel (1 ml) was added into each culture insert (n=3). Acontrol group, not exposed to any chemicals or inserts, was maintainedin parallel. After 48 h incubation, media, inserts, and hydrogels wereremoved. Each well was rinsed with phosphate buffered saline (PBS), and3 ml of a 20% CellTiter 96 Aqueous One Solution (Promega Corp., Madison,Wis., USA) in PBS was added into each well. The3-[4,5,dimethylthiazol-2-yl]-5-[3-carboxymethoxy-phenyl]-2-[4-sulfophenyl]-2H-tetrazolium(MTS-tetrazolium) compound can be metabolized by mitochondria in livingcells into a colored formazan product that is soluble in cell culturemedium. After incubating at 37° C. for 90 min, the absorbance of thesolutions was determined at 490 nm using a 96-well plate reader (SAFIRE,Tecan, Austria).

The viability of MC3T3 cells in the presence of methacrylated alginateand photocrosslinked alginate hydrogels was also investigated by aLive/Dead assay system (Sigma) comprised of fluorescein diacetate (FDA)and ethidium bromide (EB). FDA stains the cytoplasm of viable cellsgreen, while EB stains the nuclei of non-viable cells orange-red. Thestaining solution was freshly prepared by mixing 1 ml of FDA solution(1.5 mg/ml of FDA in dimethyl sulfoxide and 0.5 ml of EB solution (1mg/ml of EB in PBS) with 0.3 ml of PBS (pH 8). After 48 h incubation ofthe alginate with the cells, inserts and hydrogels were removed. Then,60 μl of staining solution was added into each well and incubated for3-5 min at room temperature. After staining, samples were examined on aninverted fluorescence microscope (ECLIPSE TE 300, Nikon, Tokyo, Japan)equipped with a digital camera (Retiga-SRV, QImaging, Burnaby, BC,Canada). Three images of each well were randomly selected for countinglive and dead cells.

Viability Assay of Cells Photoencapsulated in Alginate Hydrogels

Chondrocytes isolated from bovine articular cartilage using a previouslyreported method (Hall, A. C. et al., Am J Physiol. 270 (5 Pt1):C1300-1310, 1996) were photoencapsulated in alginate hydrogels bysuspension in methacrylated alginate solution (2 w/v % in DMEM) with0.05 w/v % photoinitiator (n=3). The cell/macromer solutions (500 μl)were pipetted into 24-well tissue culture plates (1×10⁷ cells/ml) andphotocrosslinked with UV for 10 minutes. The resulting hydrogel-cellconstructs were removed from the wells, placed in new 24-well tissueculture plates with 500 μl of fresh DMEM, and cultured in a humidifiedincubator at 37° C. with 5% CO₂ for 1 week. To determine the viabilityof the cells after encapsulation, MTS and Live/Dead assays wereperformed. After 1 week culture, the medium was discarded and thehydrogel-cell constructs were transferred into new wells after they werewashed with PBS. 500 μl of a 20% CellTiter 96 Aqueous One Solution inPBS was added into each well. After incubating at 37° C. for 90 min, thehydrogel-cell constructs were homogenized and the absorbance of thesolutions was determined at 490 nm using a 96-well plate reader. For thelive/dead assay, 20 μl of staining solution was added into each well andincubated for 3-5 min at room temperature and then stained hydrogel-cellconstructs were subjected to the fluorescence microscope.

Cell Culture on Photocrosslinked Alginate Hydrogels

MC3T3 cells were seeded on photocrosslinked alginate disks at a seedingdensity of 1×10⁴ cells/cm² and allowed to adhere for 4 h. Thephotocrosslinked alginate disks were then transferred to new platescontaining fresh media and cultured in an incubator. After 48 hincubation, alginate disks were removed from culture and stained usingthe FDA/EB to examine the viability and morphology of adhered cells.

Statistical Analysis

All quantitative data is expressed as mean±standard deviation.Statistical analysis was performed with one-way analysis of variance(ANOVA) with Tukey honestly significant difference post hoc test usingOrigin software (OriginLab Co., Northampton, Mass.). A value of p<0.05was considered statistically significant.

Results Synthesis and Characterization of Macromer and PhotocrosslinkedHydrogel

To prepare the photocrosslinked biodegradable alginate hydrogel,methacryl groups were introduced into the alginate main chains as shownin FIG. 1. Alginate was covalently reacted with various amounts of AEMA.The theoretical methacrylation of carboxyl groups of alginate variedfrom 5% to 45%. Theoretical methacrylation was calculated on the basisof the concentration of AEMA added to the alginate solution (Table 2).

TABLE 2 Methacrylation efficiency (%) of alginates and mechanicalproperties of photocrosslinked alginate hydrogels with various degreesof methacrylated macromer Theoretical Methacrylation Elasticmethacrylation^(a) efficiency^(b) modulus^(c) M_(c) Code (%) (%) (kPa)(g/mol) MAALG-4 5.66 73 —^(d) — MAALG-8 11.31 67  34.92 ± 2.48 4257MAALG-14 22.63 61 133.57 ± 9.12 1113 MAALG-25 45.26 56 170.70 ± 15.51871 ^(a)Theoretical methacrylation was calculated by modification oftotal carboxylic acid in 2% (w/v) alginate solution and molecular weightof the repeat unit (M₀ = 198). ^(b)Methacrylation efficiency wascalculated from ¹H-NMR data. ^(c)Elastic modulus was measure afterincubation in diH₂O at 37° C. for 24 hours. ^(d)It was not possible toperform a compression test on the MAALG-4.

The experimental efficiency of alginate methacrylation was calculatedfrom ¹H-NMR spectra. The intensities of proton peaks of AMEA increasedas the theoretical methacrylation of alginate increased from 5% to 45%.

The ¹H-NMR spectra of methacrylated alginate showed that the peaks ofvinyl methylene and methyl protons that were newly formed by thereaction with AEMA are located at 66.2 and 5.7, and 1.9, respectively.The completeness of photocrosslinking was also verified with ¹H-NMR.After photocrosslinking of methacrylated alginate, the disappearance ofthe peaks of vinyl methylene and shift of methyl peak to δ1.2 in the¹H-NMR spectra indicate the complete reaction of the methacrylate group.The peak of newly formed methylene proton by the photocrosslinkingappears at δ2.2.

Swelling Kinetics, Degradation, Mechanical Properties ofPhotocrosslinked Alginate Hydrogel

The size of photocrosslinked alginate hydrogel slightly increased afterone week. However, after two weeks, the size of alginate hydrogel hadrapidly increased.

Photocrosslinked alginate hydrogels were prepared with differentmethacrylation of alginate. The change of swelling ratio of thehydrogels over time was measured as this reflects changes in thephysical and chemical structure of hydrogels. Swelling ratios of thesehydrogels in diH₂O are shown in FIG. 10. All hydrogels exhibited fastswelling. The swelling of MAALG-14 and MAALG-25 reached equilibriumstage within 48 hrs, increased up to 1 and 2 weeks, respectively, andthen gradually decreased. Compared to the MAALG-14 and MAALG-25, MAALG-8exhibited much faster swelling kinetics. The swelling of MAALG-8 reacheda maximum by 24 hrs and then rapidly decreased.

The mass loss (%) of alginate hydrogels over time was determined as ameasure of degradation (FIG. 11). MAALG-8 displayed the fastestdegradation with a mass loss of 95.67% at 1 week, while MAALG-25 showedthe slowest degradation. As methacrylation of alginate increased from14% to 25%, the degradation rate of the cross-linked hydrogels decreased(FIG. 11). However, photocrosslinked alginate hydrogels withmethacrylation of 14% to 25% had relatively similar degradation rates,with complete degradation of both conditions occurring by 5 weeks.

To examine the correlation between mechanical property change anddegradation, constant strain-rate compression tests were performed onthe alginate hydrogels during the degradation study in order todetermine their elastic moduli. Representative stress-strain curves ofthe MAALG-25 alginate hydrogels during the course of degradation arepresented in FIG. 12A. It can be observed that the compressive stiffnessof alginate hydrogels decreased with degradation time as supported bythe elastic moduli results (FIG. 12B).

Cytotoxicity of Macromer and Photocrosslinked Alginate Hydrogel

The cytotoxicities of methacrylated alginate and photocrosslinkedalginate hydrogel were evaluated by measuring the mitochondrialmetabolic activity of MC3T3 cells in the presence of the biomaterialusing a standard MTS assay. The cell viability was calculated bynormalizing the absorbance of samples at 490 nm to that of the controlwithout any macromer or insert in the medium. The viability of cellscultured in the presence of macromer and photocrosslinked MAALG-25hydrogels (91% and 89%, respectively) was slightly lower than that ofcells in the cells+insert group. There was no significant difference incell viability after 48-hour culture between macromer andphotocrosslinked alginate hydrogel.

The cytotoxicity of photocrosslinked alginate hydrogel was also examinedby fluorescence staining with a Live/Dead assay, where live cells anddead cells were fluorescently labeled green and red, respectively.Almost all of MC3T3 cells were live after 48 hr exposure to the alginatehydrogel. Quantification of stained images demonstrated that there wasno significant difference in the number of live cells between thecontrol cells+insert group and the alginate hydrogel group (FIG. 13).Although the number of dead cells in the hydrogel exposed group waslarger than that of cells+insert group, the number of dead cells wasminimal compared to the number of live cells.

Chondrocyte Encapsulation

Bovine chondrocytes were photoencapsulated within MAALG-8, MAALG-14, andMAALG-25 hydrogels. The viability of the photoencapsulated chondrocytesin the alginate hydrogels was evaluated by MTS and Live/Dead assays toexamine cell survival during the photocrosslinking process and inculture. There were no significant differences in absorbances for theMTS assay between the MAALG-8, MAALG-14, and MAALG-25 conditionsimmediately following photoencapsulation (0.495±0.017, 0.499±0.019,0.496±0.023, respectively) or after 7 days in 3-D culture (0.525±0.006,0.511±0.026, 0.502±0.026, respectively). In addition, a high cellviability (>˜80%) was observed throughout all alginate hydrogelcompositions at 7 days (FIG. 14) using the Live/Dead assay.

Cell Attachment and Morphology

Few chondrocytes seeded on the surface of MAALG-25 hydrogels were ableto adhere (0.06±0.022%), and those that did adhere showed a viability of48±4.7% after 2 days of culture. All of the adherent cells exhibited aspherical morphology and none spread on the surface of the hydrogels.

Example 2 Synthesis of Methacrylated Heparin

Methacrylated heparin was prepared by reacting heparin (Mw 17000, Sigma,St. Louis, Mo. USA) with 2-aminoethyl methacrylate (AEMA, Sigma) (FIG.8). To synthesize the methacrylated heparin with methacrylation of twocarboxylic acid groups, heparin (1 g) was dissolved in a buffer solution(1% w/v, pH 6.5) of 50 mM 2-morpholinoethanesulfonic acid (MES, Sigma)containing 0.5 M NaCl. NHS (13.8 mg; Sigma) and EDC (45.1 mg; Sigma)(NHS:EDC=1:2) were added to the mixture to activate the carboxylic acidgroups of heparin. After 5 min, AEMA (21.7 mg) (molar ratio ofNHS:EDC:AEMA=1:2:1) was added to the product. The reaction wasmaintained at room temperature for 24 hours. After the reaction, themixture was precipitated by pouring into an excess of acetone, driedunder reduced pressure, and rehydrated to a 1% w/v solution in ultrapuredeionized water (diH₂O) for further purification. The methacrylatedheparin was purified by dialysis against diH₂O (MWCO 3500; SpectrumLaboratories Inc., Rancho Dominguez, Calif., USA) for 3 days, filteredthrough a 0.22 μm filter, and lyophilized until dry. To verify themethacrylation of heparin, methacrylated heparin was dissolved indeuterium oxide (Sigma) and placed in an NMR tube. The ¹H-NMR spectra ofthe methacrylated heparin was recorded on a Varian Unity-300 (300 MHz)NMR spectrometer (Varian Inc., Palo Alto, Calif., USA) usingtetramethylsilane (Sigma) as an internal standard.

Photocrosslinking

Methacrylated alginate was prepared as reported by Jeon, O. et al.,Biomaterials 30(14):2724-2734 (2009). To fabricate photocrosslinkedHP-ALG hydrogels, methacrylated alginate (0.182 g) and methacrylatedheparin (0.018 g) were dissolved in 10 ml of diH₂O or DMEM with 0.05%w/v photoinitiator (Irgacure-2959, Sigma). These solutions were placedbetween two glass plates separated by 0.75 mm spacers andphotocrosslinked with 365 nm UV light at about 1 mW/cm² for 10 minutesto form the hydrogels. Photocrosslinked hydrogel disks were createdusing a 6 mm diameter biopsy punch and placed in diH₂O or DMEM.

Mechanical Testing

The elastic moduli of the photocrosslinked HP-ALG hydrogels weredetermined by performing constant strain rate compression tests using aRheometrics Solid Analyzer (RSAII, Rheometrics Inc., Piscataway, N.J.,USA) equipped with a 10 N load cell. The photocrosslinked HP-ALGhydrogel disks were prepared as described above and maintained in diH₂Oat 37° C. After 24 hour incubation in diH₂O, swollen HP-ALG hydrogeldisks were punched once again to form 6 mm diameter disks. Theirthickness was measured using calipers, and uniaxial, unconfinedcompression tests were performed on the hydrogel disks at roomtemperature using a constant strain rate of 5%/sec. Elastic moduli ofphotocrosslinked HP-ALG hydrogels were determined from the slope ofstress vs. strain plots, limited to the linear first 5% strain of theplots.

Swelling and In Vitro Degradation of HP-ALG Hydrogels

The photocrosslinked HP-ALG hydrogels were lyophilized and dry weights(Wi) were measured. Dried hydrogel samples were immersed in 50 ml ofdiH₂O or DMEM and incubated at 37° C. to reach equilibrium swellingrate. The diH₂O or DMEM were replaced every week. Over the course of 8weeks, samples were removed, rinsed with diH₂O, and the swollen (Ws)hydrogel sample weights were measured. The swelling ratio (Q) wascalculated by Q=Ws/Wi (n=3 for each time point). After weighing theswollen hydrogels, photocrosslinked HP-ALG hydrogels were lyophilizedand weighed (Wd). The percent mass loss was calculated by (Wi−Wd)/Wi×100(n=3 for each time point).

Release Kinetics of Growth Factors

The kinetics of four different growth factors [basic fibroblast growthfactor (FGF-2), vascular endothelial cell growth factor (VEGF),transforming growth factor-beta 1 (TGF-β₁), and bone morphogeneticprotein-2 (BMP-2)] release from HP-ALG hydrogels was determined.Methacrylated alginate (27.3 mg) and methacrylated heparin (2.7 mg) weredissolved in diH₂O (1.5 ml) with 0.05% w/v photoinitiator (IrgacureD-2959, Sigma). Each of the four different growth factors (0.75 μg) wasadded to methacrylated heparin/alginate solution. After gently mixingfor 5 minutes, aliquots (300 μl) of solution were placed in 96-welltissue culture plates and photocrosslinked with 365 nm UV light at about1 mW/cm² for 10 minutes to form the hydrogels. Each photocrosslinkedhydrogel was immersed in 15-ml microcentrifuge tubes containing 10 mlphosphate buffered saline (PBS, pH 7.4) and incubated at 37° C. Atvarious time points, the supernatant was withdrawn and fresh buffer wasreplenished. The amounts of growth factors in the supernatants weredetermined with an enzyme-linked immunosorption assay (ELISA) kit(Duoset, R&D Systems, Minneapolis, Minn., USA). ELISA plates were coatedwith capture monoclonal antibodies and were blocked with bovine serumalbumin (1 w/v %) and sucrose (5 w/v %) for 1 hour. After theappropriately diluted samples were added to the ELISA plates, boundgrowth factors were detected using anti-human polyclonal antibodies.Then, streptavidin-conjugated horseradish peroxidase was added to theplates. The enzyme (peroxidase) and (substrate) tetramethylbenzidine)were added and incubated for 20 minutes. The enzyme reaction was stoppedby adding an acidic solution. The absorbance of the samples was read at450 nm using an ELISA plate reader (SAFIRE, Tecan, Austria). The amountsof growth factors were determined from a calibration curve based onknown concentrations of growth factors.

Bioactivity Assay of Growth Factors In Vitro

The bioactivity of BMP-2 released from HP-ALG hydrogels in vitro wasassessed by determining its ability to stimulate the ALP activity ofMC3T3 preosteoblasts cultured in DMEM containing 10% (v/v) fetal bovineserum at 37° C. with 5% (v/v) CO₂. One μg of BMP-2 per hydrogel wasloaded into photocrosslinked HP-ALG hydrogels. Cells (3×10⁴) were platedin each well of six-well tissue culture plates and the BMP-2 loadedhydrogels were fixed on culture inserts (TRANSWELL, Corning Inc.). Themedium was changed every three days. At predetermined time points, theALP activities were measured. At each time point, cells were lysed byrepeating freeze/thaw cycle three times, and the lysates were cleared bycentrifugation for 10 minutes at 13000 RPM using an ultracentrifuge. 25μl of supernatant was incubated with 150 μl of ALP substrate containingp-nitrophenylphosphate (pNPP, Sigma) at 37° C. for 30 minutes. Thereaction was stopped by adding 25 μl of 3 N NaOH to the substratereaction solution. The absorbance of the samples was read at 405 nmusing an ELISA plate reader. Each ALP activity measurement wasnormalized by the protein content, which was measured by the BCA proteinassay reagent (Pierce Chemical, Rockford, Ill., USA).

The bioactivities of VEGF released from HP-ALG hydrogels in vitro bydetermining its ability to stimulate the proliferation of humanumbilical vein endothelial cells (HUVECs) cultured in endothelial cellbasal medium-2 (EBM-2, Cambrex Bio Science Walkersville, Inc.,Walkersville, Md., USA) with 2% (v/v) fetal bovine serum. One μg of VEGFwas loaded into HP-ALG hydrogels. Cells (1×10⁵ per well) were plated ineach well of six-well tissue culture plates and the VEGF-loadedhydrogels were fixed on culture inserts (TRANSWELL). The medium waschanged every three days. On weeks 1, 2 and 3, cell numbers weremeasured using a hemacytometer. One μg of VEGF was loaded into heparinlacking alginate hydrogels served as a comparative group. As acomparative group, VEGF in free form was added into a HUVEC culture inEBM-2 at the concentration of 100 ng/ml. HUVECs cultured in endothelialcell growth medium (EGM-2) or EMB-2 without VEGF served as a positive ornegative control, respectively.

In Vivo Bone Formation

All animal procedures were carried out in accordance with a protocolapproved by the Institutional Animal Care and Usage Committee. Fifteenmale scid mice (ICRSC, 3-4 weeks old; Taconic City, USA) were dividedinto three groups and anesthetized with xylazine (20 mg/kg) and ketamine(100 mg/kg). Small incisions were made on the dorsal skins of mice. Twopouches per animal were made by blunt dissection in subcutaneous sites,and BMP-2 (1 μg)-loaded photocrosslinked HP-ALG hydrogel disks wereimmediately implanted into the pouches (n=10). Subsequently, the skinwas closed with 6-0 silk sutures (Ethicon, Lenneke Marelaan, Belgium).Eight weeks after the implantation, the rats were sacrificed and theimplants were retrieved. Five implants were used for histologicalanalysis, and the others were used for a calcium content assay. Thehistological specimens were fixed in formalin, embedded in paraffin,sectioned at a thickness of 4 μm, and examined with hematoxylin andeosin staining and Goldner's trichome staining. The bone formation areawas measured using an image analysis system (KS400, Zeiss, Munich,Germany) coupled to a light microscope. The bone formation area wasexpressed as the percentage of bone area in total cross-sectional area[(bone area/total area)×100%]]. The amount of calcium deposited in theimplants was measured as previously described.

Statistical Analysis

All quantitative data were expressed as the mean±standard deviation.Statistical analysis was performed with one-way analysis of variance(ANOVA) with Tukey honestly significantly difference post hoc test usingOrigin software. A value of p<0.05 was considered statisticallysignificant.

Synthesis and Characterization of Methacrylated Heparin andPhotocrosslinked HP-ALG Hydrogel

To prepare photocrosslinked HP-ALG hydrogel, methacrylates wereintroduced into the heparin main chains as shown in FIG. 8. The ¹H-NMRspectra of methacrylated heparin exhibit peaks of vinyl methylene andmethyl protons that were newly formed by the reaction with AEMA arelocated at δ6.2, 5.7, and 1.9, respectively.

DMEM or diH₂O-equilibrated HP-ALG and alginate hydrogel disks exhibitedno significant differences in gross morphologies or size change betweentwo groups after 24 hrs equilibrium.

Elastic Moduli, Swelling Kinetics, and Degradation of thePhotocrosslinked HP-ALG Hydrogel

To compare mechanical properties between photocrosslinked HP-ALGhydrogel and alginate hydrogel, constant strain-rate compression testswere performed on the alginate hydrogels after 24 hours equilibrium inDMEM or diH₂O. There was no significant difference in compressivemodulus between the two groups as shown in FIG. 15A.

The swelling ratio change of the alginate hydrogels measured over timereflects changes in their physical and chemical structure. Swellingratios of these hydrogels in DMEM are shown in FIG. 15B. Both hydrogelsdisplayed rapid swelling after 2 hours, and slightly increased up to 8weeks. The swelling of the photocrosslinked HP-ALG and normal alginatehydrogels reached equilibrium stage within 48 hours, increased up to 1and 2 weeks, respectively, then gradually decreased in diH₂O (FIG. 15C).Compared to the normal alginate hydrogel, photocrosslinked HP-ALGhydrogel exhibited slower swelling kinetics. The mass loss (%) ofphotocrosslinked HP-ALG hydrogels over time was determined as a measureof degradation. The mass loss of the photocrosslinked HP-ALG and normalalginate hydrogels were fairly similar regardless of methacrylatedheparin addition in DMEM (FIG. 15D). However, in diH₂O, photocrosslinkedHP-ALG hydrogels showed slower mass loss than alginate hydrogel (FIG.15E).

Release Kinetics

The release profiles of growth factors from photocrosslinked HP-ALGhydrogels were determined using ELISA and compared with those fromphotocrosslinked alginate hydrogel. The release of growth factors fromthe photocrosslinked alginate hydrogels was more rapid than that ofphotocrosslinked HP-ALG hydrogel delivery system. Almost all of thegrowth factors were released from the photocrosslinked alginatehydrogels within the first 7 days. In contrast, the release of growthfactors from the photocrosslinked HP-ALG hydrogels was slower andsustained over three weeks (FIGS. 16A-D).

Bioactivity of Growth Factors Released from Photocrosslinked HP-ALGHydrogel

To determine whether the growth factors released from photocrosslinkedHP-ALG hydrogel are bioactive, the biological activity of the VEGF wasevaluated by measuring its ability to stimulate the growth of HUVECs inmedium containing delivery system. The HUVECs showed the lowest cellgrowth in the basal medium without VEGF. VEGF-loaded normal alginatehydrogel slightly improved HUVEC growth compared to the basal medium.Moreover, VEGF-loaded photocrosslinked HP-ALG hydrogel significantlyimproved HUVEC growth compared to VEGF-loaded normal alginate hydrogel.The cell growth for 3 weeks in VEGF-loaded photocrosslinked HP-ALGhydrogel was not different from that in the EGM (FIG. 17A). Thisindicates that the VEGF released from photocrosslinked HP-ALG hydrogelfor up to 3 weeks was bioactive.

The biological activity of the BMP-2 released from HP-ALG hydrogel wasalso evaluated by measuring its ability to stimulate the ALP activity ofMC3T3 preosteoblast culture in a medium containing the delivery system.The ALP activity of MC3T3 cells did not increase over the entire cultureperiod for photocrosslinked HP-ALG hydrogel without BMP-2. The additionof BMP-2 in a free form to the culture medium enhanced the ALP activityover the entire culture period. The ALP activities for BMP-2-loadedphotocrosslinked HP-ALG hydrogel were not different from that of BMP-2addition to the medium (FIG. 17B). This indicates that the BMP-2released from photocrosslinked HP-ALG hydrogel was bioactive.

Ectopic Bone Formation

The delivery of BMP-2 using photocrosslinked HP-ALG hydrogel enhancedectopic bone formation compared to the delivery of BMP-2 usingphotocrosslinked alginate hydrogel. Histological analysis showed noevidence of bone formation in the photocrosslinked HP-ALG hydrogelwithout BMP-2. Implantation of BMP-2-loaded photocrosslinked alginatehydrogel induced moderate bone formation. Importantly, BMP-2-loadedphotocrosslinked HP-ALG hydrogel induced bone formation to a muchgreater extent than did BMP-2-loaded photocrosslinked alginate hydrogel(FIGS. 18A-B).

The bone formation area and calcium deposition in the implants werehigher in the BMP-2-loaded photocrosslinked HP-ALG hydrogel than in theBMP-2-loaded photocrosslinked alginate hydrogel at eight weeks afterimplantation (FIGS. 19A-B). The bone formation area and calciumdeposition in the photocrosslinked HP-ALG hydrogel without BMP-2 werefar less than those in the other groups (FIGS. 19A-B).

Example 3 Preparation of RGD-Modified Methacrylated Alginate

RGD-modified methacrylated alginate was synthesized in a two-stepreaction utilizing standard carbodiimide chemistry (FIG. 3). Lowmolecular weight of sodium alginate (37,000 g/mol) was prepared byirradiating Protanal LF 20/40 (196,000 g/mol, FMC Biopolymer,Philadelphia, Pa., USA) at a gamma dose of 5 Mrad. Twenty-five percentactual methacrylation of alginate carboxylic groups was performed asdescribed in Jeon, O. et al., Biomaterials 30(14):2724-2734 (May 2009).Methacrylated alginate solutions (1 w/v %) were prepared with 50 mM of2-(N-morpholino)ethanesulfonic acid hydrate (MES, Sigma, St. Louis, Mo.,USA) buffer solution containing 0.5 M NaCl (Sigma) at pH 6.5, andsequentially mixed with N-hydroxysuccinimide (NHS, Sigma) and1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide (EDC, Sigma). The molarratio of NHS to EDC was 0.5:1.0, and the weight ratio of EDC tomethacrylated alginate was 1.0:20.7. The amino acid peptide sequence ofSEQ ID NO: 1 (Commonwealth Biotechnologies, Richmond, Va.) was added tothe methacrylated alginate solution at a weight ratio of 10 mg/gmethacrylated alginate. After reacting for 24 h at 4° C., the reactionwas stopped by addition of hydroxylamine (0.18 mg/ml, Sigma), and thesolution was purified by dialysis against diH₂O (MWCO 3500; SpectrumLaboratories Inc., Rancho Dominguez, Calif., USA) for 3 days, treatedwith activated charcoal (0.5 mg/100 ml, 50-200 mesh, Fisher, Pittsburgh,Pa.) for 30 min, filtered (0.22 μm filter), and lyophilized. Controlmethacrylated alginate was prepared in same manner but without thepresence of peptide.

Characterization of RGD-Modified Methacrylated Alginate

To verify the RGD-modification of the methacrylated alginate, an ¹H-NMRspectra of RGD-modified methacrylated alginate was recorded.RGD-modified methacrylated alginate was dissolved in deuterium oxide(Sigma) and placed in an NMR tube. The ¹H-NMR spectrum of theRGD-modified methacrylated alginate was recorded on a Varian Unity-300(300 MHz) NMR spectrometer (Varian Inc., Palo Alto, Calif., USA) usingtetramethylsilane (TMS) as internal standard. To analyze the degree ofpeptide modification, a ninhydrin assay was performed. Briefly,RGD-modified methacrylated alginate was dissolved in 5 ml of 1 M sodiumacetate buffer (pH 5) and ninhydrin reagent was added. The mixture waskept in boiling water for 20 min. After incubation, 75 ml of adiH₂O/absolute ethanol mixture (1/1, v/v) was added and the reactionmixture was cooled to room temperature for 2 h in complete darkness.Ninhydrin reacted with free amino groups and created a water-solubleblue compound. The amount of free amino groups in the RGD-modifiedmethacrylated alginate was determined by measuring the UV absorbance ofthe supernatant at 570 nm. Methacrylated alginate and glycine were usedas the control and the standard, respectively.

Photocrosslinking

To fabricate photocrosslinked RGD-modified alginate or unmodifiedhydrogels, RGD-modified methacrylated alginate (0.2 g) or unmodifiedmethacrylated alginate (0.2 g) were dissolved in DMEM or diH₂O (10 ml)with 0.05% w/v photoinitiator (Irgacure D-2959, Sigma) for ultimateplacement in DMEM or diH₂O, respectively. The alginate solutions wereinjected between two glass plates separated by 0.75 mm spacers andphotocrosslinked with 365 nm UV light (Model ENF-260C, Spectroline,Westbury, N.Y.) at about 1 mW/cm² for 10 min to form the hydrogels.Photocrosslinked hydrogel disks were created using a 6 mm diameterbiopsy punch and placed in DMEM or diH₂O for swelling and degradationstudies, mechanical testing, and culture of cells on the hydrogelsurfaces.

Swelling and Degradation of Hydrogels

The photocrosslinked RGD-modified or unmodified alginate hydrogels werelyophilized and dry weights (W_(i)) were measured. Dried hydrogelsamples were immersed in 50 ml of DMEM or diH₂O and incubated at 37° C.to reach equilibrium swelling state. The DMEM or diH₂O was replacedevery three days. Over the course of 8 weeks, samples were removed fromthe DMEM or diH₂O, and the swollen (W_(s)) hydrogel sample weights weremeasured. The swelling ratio (Q) was calculated by Q=W_(s)/W_(i) (N=3for each time point). After weighing the swollen hydrogel samples, thesamples were lyophilized and weighed (W_(d)). The percent mass loss wascalculated by (W_(i)−W_(d))/W_(i)×100 (N=3 for each time point).

Mechanical Testing

The elastic moduli of the photocrosslinked RGD-modified and unmodifiedalginate hydrogels were determined by performing constant strain ratecompression tests using a Rheometrics Solid Analyzer (RSAII, RheometricsInc., Piscataway, N.J., USA) equipped with a 10 N load cell. Thephotocrosslinked RGD-modified and unmodified alginate hydrogel diskswere prepared as described in the photocrosslinking section andmaintained in DMEM or diH₂O at 37° C. After 24 h incubation, swollenalginate hydrogel disks were punched once again to form 6 mm diameterdisks, their thickness was measured using calipers, and uniaxial,unconfined compression tests were performed on the hydrogel disks atroom temperature using a constant crosshead speed of 5%/sec. Elasticmoduli of photocrosslinked alginate hydrogels were determined from theslope of stress vs. strain plots, limited to the first 5% of strain(N=3).

Cell Culture on the Alginate Hydrogels

Chondrocytes (passage number 2) isolated from bovine articular cartilageas reported by Paige, K. T. et al., Plast Reconstr Surg. 96(6):1390-1398(November 1995) were seeded on photocrosslinked RGD-modified orunmodified alginate hydrogel disks in DMEM containing 10% fetal bovineserum (FBS) at a seeding density of 1×10⁴ cells/cm² in 24-well tissueculture plates and allowed to adhere for 4 hrs in a humidified incubatorat 37° C. with 5% CO₂. The photocrosslinked RGD-modified or unmodifiedalginate disks were then transferred to new plates containing freshmedia and cultured. The viability and morphology of adhered cells on theRGD-modified or unmodified alginate disks were examined by Live/Deadassay system (Sigma) comprised of fluorescein diacetate (FDA) andethidium bromide (EB). FDA stains the cytoplasm of viable cells green,while EB stains the nuclei of non-viable cells orange-red. The stainingsolution was freshly prepared by mixing 1 ml of FDA solution (1.5 mg/mlof FDA in dimethyl sulfoxide) (Research Organics Inc., Cleveland, Ohio)and 0.5 ml of EB solution (1 mg/ml of EB in PBS) with 0.3 ml of PBS (pH8). At predetermined time points, 20 μl of staining solution was addedinto each well and incubated for 3-5 min at room temperature, and thenstained hydrogel-cell constructs were examined using fluorescencemicroscopy.

Encapsulation of Chondrocytes

Chondrocytes (passage number 2) were photoencapsulated in RGD-modifiedor unmodified alginate hydrogels by suspension in RGD-modified orunmodified methacrylated alginate solution (2% w/v in DMEM) with 0.05%w/v photoinitiator. The cell/macromer solutions (300 μl) were pipettedinto 96-well tissue culture plates (1×10⁷ cells/ml) and photocrosslinkedwith UV for 10 minutes. The resulting hydrogel-cell constructs wereremoved from the wells, placed in new 24-well tissue culture plates with1 ml of fresh DMEM, and cultured in a humidified incubator at 37° C.with 5% CO² for 6 weeks. The viability of encapsulated chondrocytes inthe photocrosslinked RGD-modified alginate hydrogels was investigated bya Live/Dead assay (N=3 for each time point).

Biochemical Assays for DNA Content and GAG Production

At each time point, hydrogel-cell constructs were removed from media,homogenized and digested in papain buffer solution (Sigma, 25 μg/mlpapain, 2 mM cystein, 50 mM sodium phosphate, 2 mM EDTA, pH 6.5 innuclease-free water) at 65° C. for 3 h. Hoechst 33258 dye (0.1 μg/ml innuclease free water, Acros Organics, Morris Plains, N.J.) was used forthe DNA assay as described by Solorio, L. D. et al., J Biomed Mater ResA. (Mar. 25, 2009). Calf Thymus DNA standards (Rockland Immunochemicals,Gilbertsville, Pa.) were prepared with 0-4 μg/ml DNA in nuclease freewater. After the centrifugation of papain-digested samples, 100 μl ofsupernatants was mixed with 100 μl of the prepared dye solution.Fluorescence intensity of the dye-conjugated DNA solution was measuredin 96-well plates on a fluorometer (358 nm excitation and 452 nmemission, SAFIRE, Tecan, Austria), and the DNA content was calculatedfrom a standard curve generated with calf thymus DNA. GAG content wasmeasured using the standard dimethylmethylene blue (DMMB, Sigma) assayin 96-well plates as described by Enobakhare, B. O. et al., AnalBiochem. 243(1):189-191 (Dec. 1 1996). In each well, 50 μl of digest wasmixed with 250 μl of dye containing 16 mg/L DMMB and 3.04 g/L glycine(pH 1.5). The absorbance was read at 595 nm using the plate reader(SAFIRE). Chondroitin-6-sulfate (Sigma) from shark cartilage was used toconstruct the standard curve.

Encapsulation of Chondrocytes and TGF-β₁

An in vitro TGF-β₁ release study was performed to examine the timecourse of TGF-β₁ release from photocrosslinked RGD-modified alginatehydrogels. TGF-β₁ (0.75 μg, PeproTech, Rochy Hill, N.J.) was added toRGD-modified methacrylated alginate solution (1.5 ml, 2 w/v % in diH₂O).After gently mixing for 5 min, aliquots (300 μl) of solution were placedin 96-well tissue culture plates and photocrosslinked with 365 nm UVlight at ˜1 mW/cm² for 10 min. Each photocrosslinked hydrogel wasimmerged in 15-ml microcentrifuge tubes containing 10 ml PBS andincubated at 37° C. (N=5). At predetermined time points, the supernatantwas withdrawn and fresh buffer was replenished. The amount of TGF-β₁ inthe supernatants was determined using an enzyme-linked immunosorptionassay (ELISA) kit (Human TGF-β₁ Duoset, R&D Systems, Minnepolis, Minn.,USA). TGF-β₁ loaded photocrosslinked unmodified alginate hydrogels wasused as a comparative group (N=5).

Photocrosslinked RGD-modified unmodified alginate hydrogel-cellconstructs containing TGF-β₁ (100 ng/hydrogel) were prepared in 96-welltissue culture plates as described above, removed from the wells, placedin new 24-well tissue culture plates with 1 ml of fresh DMEM, andcultured in a humidified incubator at 37° C. with 5% CO₂ for 6 weeks(N=3). As a control, hydrogel-cell constructs without TGF-β₁ werecultured in DMEM containing 10 ng/ml TGF-β₁ (N=3). The medium waschanged every three days. At predetermined time points, Live/Dead, GAG,and DNA assays were performed as described above.

Statistical Analysis

All quantitative data is expressed as mean±standard deviation.Statistical analysis was performed with one-way analysis of variance(ANOVA) with Tukey honestly significant difference post hoc test usingOrigin software (OriginLab Co., Northampton, Mass.). A value of p<0.05was considered statistically significant.

Preparation and Characterization of Cell Adhesion Peptide-ModifiedMethacrylated Alginate

While alginate hydrogels provide space and mechanical support for tissueregeneration, in their native form they do not provide a mechanism forencapsulted cells to interact and receive important signalinginformation via adhesion. To partially mimic the cell adhesion capacityof native ECM, a widely used approach is to chemically incorporatespecific cell adhesion ligands, such as the ubiquitous cell adhesionpeptide sequence of SEQ ID NO: 1 (i.e., RGD) (Rowley, J. A. et al.,Biomaterials 20(1):45-53 (January 1999); Burdick, J. A. et al.,Biomaterials 23(22):4315-4323 (November 2002); Massia, S. P. et al.,Anal Biochem. 187(2):292-301 (June 1990)), which is present in numerousECM molecules such as fibronectin, collagen and laminin (Bernard, M. P.et al., Biochemistry-Us 22(5):1139-1145 (1983); Pierschbacher, M. D. etal., J Biol Chem. 257(16):9593-9597 (1992)). In this study, peptidescontaining the RGD sequence were covalently coupled onto themethacrylated alginate main chains in order to prepare RGD-modifiedphotocrosslinkable and biodegradable alginate hydrogels as shown in FIG.3. Following alginate methacrylation, the remaining carboxylic acidfunctional groups along the alginate backbone offered the potential forcovalent modification with RGD-containing cell adhesion ligands. ¹H-NMRspectra of RGD-modified methacrylated alginate macromers exhibit protonpeaks that were newly formed by the reaction with peptide, which arelocated at δ2.75 and 1.7. The proton peaks of vinyl methylene and methylprotons of AEMA are located at δ6.2 and 5.7, and 1.9, respectively. Theconjugated RGD concentration was 3.76±0.24 mg/g methacrylated alginateas measured by ninhydrin assay.

Elastic Moduli, Swelling Kinetics, and Degradation of the RGD-ModifiedPhotocrosslinked Alginate Hydrogel

To examine whether peptide modification has an effect onphotocrosslinked hydrogel mechanical properties, constant strain-ratecompression tests were performed after 24 hrs equilibration in DMEM.Representative stress-strain curves of the RGD-modified and unmodifiedphotocrosslinked alginate hydrogels (FIG. 20A) are similar in shape.There was no significant difference in compressive modulus between thetwo groups (FIG. 20B). These results provide evidence that adhesionpeptide modification of methacrylated alginate does not substantiallyaffect the crosslinked structure of photocrosslinked alginate hydrogelsbecause the compressive mechanical response of the hydrogels wasunaltered (Zosel, A. et al., Macromolecules 26(9):2222-2227 (April1993)). In addition, the swelling ratio changes and degradation profilesin DMEM were measured to further evaluate whether RGD modification hasan effect on the crosslinked structure of the alginate hydrogels. BothRGD-modified and unmodified photocrosslinked alginate hydrogels showedrapid swelling for the first day (FIG. 20C). The swelling of both groupsthen gradually increased over the course of 8 weeks. The hydrogelsexhibited similar swelling kinetics for 4 weeks, and the swelling ratioof the RGD-modified photocrosslinked alginate hydrogels was onlyslightly higher than that of unmodified alginate hydrogel after 8 weeks.These results also indicate that RGD-modification of methacrylatedalginate does not substantially affect the macromolecular structure ofphotocrosslinked alginate hydrogel over time. The mass loss (%) ofalginate hydrogels over time was determined as a measure of degradation(FIG. 20D). The degradation of photocrosslinked RGD-modified alginatehydrogels was slightly faster than that of unmodified alginate hydrogelsat 2 and 4 weeks, but there was no difference between the two groups at8 weeks. These properties were also quantified for the hydrogels inultrapure deionized water (diH₂O). (FIGS. 21A-B).

Characterization of 2D and 3D Chondrocyte Cultures

Bovine chondrocytes were seeded onto RGD-modified and unmodifiedphotocrosslinked alginate hydrogels to evaluate if the peptidemodification would enhance chondrocyte adhesion and proliferation.Chondrocytes adhered to the surfaces of RGD-modified hydrogels by 4hours (data not shown) and exhibited substantial spreading by 7 days.Few chondrocytes seeded on the surfaces of unmodified hydrogels wereable to adhere, and those that did remained rounded through 7 days.These results indicate that the chondrocyte cell adhesion and spreadingwere mediated by the immobilized adhesion ligands (Alsberg, E. et al., PNatl Acad Sci USA 99(19):12025-12030 (Sep. 17 2002); Ji, J. et al.,Biomaterials 25(10):1859-1867 (May 2004); Park, K. M. et al.,Macromolecular Research 16(6):517-523 (August 2008)). Chondrocytes werethen photoencapsulated within RGD-modified or unmodifiedphotocrosslinked alginate hydrogels to provide a 3D culture environmentwhich more closely resembles native cartilage tissue. To examine cellsurvival during the photocrosslinking process and during culture, theviability of the photoencapsulated chondrocytes in the alginatehydrogels was evaluated by a Live/Dead assay. High cell viability wasobserved throughout all alginate hydrogel compositions for 6 weeks.Chondrocytes in native cartilage are located in lacunae surrounded byECM (Verdonk, P.C. M. et al., Osteoarthr Cartilage 13(7):548-560 (July2005). ECM-cell interactions promote chondrocyte aggregation (Cao, L. etal., Matrix Biol. 18(4):343-355 (August 1999), reduce the level ofchondrocyte apoptosis (Shakibaei, M. et al., J Biol Chem.276(16):13289-13294 (April 2001), and are essential for chondrocyteproliferation, differentiation, survival, and maintenance ofchondrogenic activity (Svoboda, K. K. H., Microsc Res Techniq.43(2):111-122 (October 1998); Grashoff, C. et al., Embo Rep.4(4):432-438 (April 2003). The DNA content of RGD-modified hydrogelgroup was significantly greater than that of unmodified hydrogel group,indicating that RGD modification promoted chondrocyte proliferation inalginate hydrogels. DNA content also significantly increased over timein both groups. After 2 and 4 weeks of culture, chondrocytesphotoencapsulated in the RGD-modified alginate hydrogels producedsignificantly more glycosaminoglycan (GAG), one of the majorconstituents of cartilage ECM (Wang, D. A. et al., Nat. Mater.6(5):385-392 (May 2007), normalized to DNA content as compared to cellsin the unmodified alginate hydrogels. No difference was present at 6weeks. This result demonstrates that regulating chondrocyte-ECMinteractions through controlled integrin-adhesion ligand signalingpromotes and accelerates the chondrogenic activity of chondrocytesencapsulated in the photocrosslinked alginate hydrogels. This positiveeffect on chondrogenesis occurred in the absence of any specific solublechondrogenic factors other than those present in the serum used.

Encapsulation of Chondrocytes and TGF-β₁

Growth factors such as TGF-β₁, which is a member of the TGF-βsuperfamily, are an important part of the soluble biochemical signalingenvironment which regulate chondrogenesis during development and promotechondrocyte-specific cellular function and cartilaginous ECM production(Kim, S. E. et al., J Control Release 91(3):365-374 (September 2003).When chondrocytes are cultured in a three-dimensional environment (e.g.,aggregate culture or in a hydrogel), TGF-β₁ stimulates synthesis ofcartilaginous ECM components such as GAG and collagen type II (Lee, J.E. et al., Biomaterials 25(18):4163-4173 (August 2004). Therefore, theeffect of adhesion ligand modification on the responsiveness ofphotoencapsulated chondrocytes to TGF-β₁ delivered either exogenously inthe cell culture media (10/ng/ml every 3 days for 6 weeks=140 ng total)or released from the alginate hydrogel itself (100 ng total perhydrogel) was investigated. TGF-β₁ was released from photocrosslinkedRGD-modified and unmodified alginate hydrogels for 4 days at an averagerelease rate of 23.6 and 25.4 ng/hydrogel/day, respectively (FIG. 22).The photoencapsulated chondrocytes exposed to TGF-β₁ in the alginatehydrogels exhibited high cell viability as observed throughout allconstruct compositions for 6 weeks using a Live/Dead assay. Chondrogenicactivity of the chondrocytes as measured by GAG/DNA content revealedthat TGF-β₁, delivered in the media or from the hydrogels, promoted GAGproduction per cell (FIG. 23). Similar levels of GAG production weremeasured when growth factor was delivered via either manner at all timepoints, even though the release of TGF-β₁ from the alginate hydrogelswas completed within one week and the total amount released was lessthan that presented exogenously. However, no significant difference inchondrogenic activity was revealed between cells encapsulated inunmodified or RGD-modified alginate hydrogels which were exposed toTGF-β₁ either in the media or from the hydrogels at all time points.These results indicate that for the specific conditions examined in thisstudy (i.e., peptide type and concentration, TGF-β₁ concentration),chondrogenic activity of the chondrocytes in the hydrogels was morestrongly influenced by the presence of growth factor than controlledECM-cell interactions.

From the above description of the invention, those skilled in the artwill perceive improvements, changes and modifications. Suchimprovements, changes, and modifications are within the skill of the artand are intended to be covered by the appended claims. All patents andpublications identified herein are incorporated by reference in theirentirety.

1. A photocrosslinked biodegradable hydrogel comprising: a plurality ofnatural polymer macromers cross-linked with a plurality of cross-linksthat are degradable in vivo, the hydrogel being cytocompatible and, upondegradation, producing substantially non-toxic products.
 2. Thephotocrosslinked biodegradable hydrogel of claim 1, the cross-linksbeing degraded by hydrolysis of the cross-links.
 3. The photocrosslinkedbiodegradable hydrogel of claim 1, the cross-links comprisinghydrolyzable acrylate cross-links.
 4. The photocrosslinked biodegradablehydrogel of claim 3, the hydrolyzable acrylate cross-link comprising atleast one of a hydrolyzable ester or hydrolyzable amide.
 5. Thephotocrosslinked biodegradable hydrogel of claim 3, the hydrolyzableacrylate cross-link having Formula I:

wherein R₁ and R₂ are the same or different natural polymer macromers.6. The photocrosslinked biodegradable hydrogel of claim 4, thehydrolyzable acrylate cross-link comprising a hydrolyzable methacrylatecross-link having Formula II:

wherein R₁ and R₂ are the same or different natural polymer macromers.7. The photocrosslinked biodegradable hydrogel of claim 6, each of R₁and R₂ comprising an alginate main chain.
 8. The photocrosslinkedbiodegradable hydrogel of claim 6, R₁ comprising an alginate main chainand R₂ comprising a heparin main chain.
 9. The photocrosslinkedbiodegradable hydrogel of claim 1 further including at least one celldispersed within or on the photocrosslinked biodegradable hydrogel. 10.The photocrosslinked biodegradable hydrogel of claim 9, the at least onecell comprising a progenitor cell.
 11. The photocrosslinkedbiodegradable hydrogel of claim 9 further including at least oneattachment molecule to couple the at least one cell to thephotocrosslinked biodegradable hydrogel.
 12. The photocrosslinkedbiodegradable hydrogel of claim 11, the at least one attachment moleculebeing acrylated or methacrylated.
 13. The photocrosslinked biodegradablehydrogel of claim 1 further including a bioactive agent that modulates afunction and/or characteristic of a cell.
 14. The photocrosslinkedbiodegradable hydrogel of claim 13, the bioactive agent including anyagent capable of regulating a cell behavior and/or targeting a specificdisease state.
 15. The photocrosslinked biodegradable hydrogel of claim13, release of the bioactive agent from the hydrogel being dependent onthe size and composition of the hydrogel.
 16. The photocrosslinkedbiodegradable hydrogel of claim 1 further including a carrier materialdispersed on or within the hydrogel, the carrier material including abioactive agent for modulating a function and/or characteristic of acell.
 17. The photocrosslinked biodegradable hydrogel of claim 1, thehydrogel comprising a microparticle or a nanoparticle.
 18. Aphotocrosslinked biodegradable hydrogel comprising: a plurality ofnatural polymer macromers cross-linked with a plurality of hydrolyzableacrylate cross-links that can be hydrolyzed in vivo, the hydrogel beingcytocompatible and, upon degradation, producing substantially non-toxicproducts.
 19. The photocrosslinked biodegradable hydrogel of claim 18,the hydrolyzable acrylate cross-link comprising at least one of ahydrolyzable ester or hydrolyzable amide.
 20. The photocrosslinkedbiodegradable hydrogel of claim 18 further including at least one celldispersed within or on the photocrosslinked biodegradable hydrogel. 21.The photocrosslinked biodegradable hydrogel of claim 20 furtherincluding at least one attachment molecule to couple the at least onecell to the photocrosslinked biodegradable hydrogel.
 22. Thephotocrosslinked biodegradable hydrogel of claim 18 further including abioactive agent that modulates a function and/or characteristic of acell.
 23. The photocrosslinked biodegradable hydrogel of claim 22,release of the bioactive agent from the hydrogel being dependent on thesize and composition of the hydrogel.
 24. The photocrosslinkedbiodegradable hydrogel of claim 18 further including a carrier materialdispersed on or within the hydrogel, the carrier material including abioactive agent for modulating a function and/or characteristic of acell.
 25. The photocrosslinked biodegradable hydrogel of claim 18, thehydrogel comprising a microparticle or a nanoparticle.
 26. A method forforming a photocrosslinked biodegradable hydrogel, the methodcomprising: reacting at least one crosslinkable acryl group with atleast one natural polymer macromer to form a mixture comprising aplurality of acrylated macromers, the acryl group including at least onehydrolyzable linkage; contacting the mixture with a photoinitiator toinitiate cross-linking of the macromers; and exposing the mixture to alight source to initiate cross-linking of the macromers and thereby forma plurality of natural polymer macromers cross-linked with a pluralityof hydrolyzable acrylate cross-links.
 27. The method of claim 26, thehydrolyzable acrylate cross-link comprising at least one of ahydrolyzable ester or hydrolyzable amide.
 28. The method of claim 26,further dispersing at least one cell within or on the photocrosslinkedbiodegradable hydrogel.
 29. The method of claim 20, the hydrogel furtherincluding at least one attachment molecule to couple the at least onecell to the photocrosslinked biodegradable hydrogel.
 30. The method ofclaim 18, further dispersing a bioactive agent in the hydrogel thatmodulates a function and/or characteristic of a cell.
 31. A method forpromoting tissue growth in a subject comprising: administering aphotocrosslinked biodegradable hydrogel to a target site in the subject,the hydrogel comprising a plurality of natural polymer macromerscross-linked with a plurality of hydrolyzable acrylate cross-links andat least one cell dispersed on or within the hydrogel, the hydrogelbeing cytocompatible and producing substantially non-toxic products upondegradation.